• They not compatible with shelf or rack configurations.  Generally, HID lights are intended to be suspended 4' or higher from their target.  For taller plants, this is good.  However, this presents a sub-optimal space arrangement for seedlings and plants of smaller stature. 
• HID produce a great deal of heat.  Plants too close to the lamps can be burned from the excess heat.  The lights heat up grow tents.  Most have vents built around the built which work with blower fans to attempt to remove heat from the grow tent.
• HID efficiency doesn't stack up against many modern LED units
• Light output drops over time.  Most lamps are specified to drop to 70% of less of their initial levels at 10,000 hours of use.  This means bulb replacements required on a yearly basis
• HID explosions are rare but when they occur, it can be dangerous (high pressure hazardous gas inside a glass enclosure).  I've been in a research lab where an HID microscope source blew and I am glad I wasn't in the room.
• Not waterproof.  For horticultural units, I've noticed an effort to make the electronics enclosed but no IP ratings are given.  When the unit is on, the heat of the HID bulb likely keeps the electronics dry.

LEDs are the most promising technology for the future of lighting.  Over the last decade, LED performance has rapidly improved while cost has been driven down.  The modular form factor has given engineers and manufacturers more flexibility in design than any other lighting technology.  Thus, we are currently seeing a revolution in applications and products. 

LEDs function through the principle of electroluminescence.  An applied voltage causes photons to be emitted as electrons "jump" across the p-n junction formed between semi-conductor materials.  The electrons (and their partner holes) have a range of energies across the junction, thus the photons emitted as they cross also have a range of energies/wavelengths.  In the visible, most LED spectra  have a full width half max (FWHM) of around 30nm-50nm.  This isn't quite as monochromatic as a laser; however, to the human eye it looks like a very well defined color.  It should be noted that there's a limited number of semi-conductor material combinations and not all colors can be efficiently created directly.  Phosphors can be used to "spread" photons to longer wavelengths.  Many white light LEDs rely on phosphors over the top of blue LEDs.  Therefore, a signature blue peak is often superimposed over a broader white spectra.  Note that most units have a mixture of LED modules and the type and ratio will determine the spectra of the unit.  

Here's 2 different white grow light spectra above.  Below see a spectra with LEDs only targeting the chlorophyll peak absorption ranges.  This lighting is affectionately referred to as "blurple".

LED form factor and light output can be tailored for the application.  The most common LED bulbs on the market are those intended to replace fluorescent and incandescent bulbs in residential lighting.  These bulbs are of limited value as grow lights except for just a few plants placed very close to them.  Many of these bulbs aren't specified in terms of watts or lumens but as the "perceptible equivalent" to other light sources.  The "perceptible equivalent" metric is muddled because the human eye's perception to brightness is highly dependent on spectra.  Even when spectra are the same, the eye responds to intensity differences more logarithmic than linear.  Thus, a 50% difference in lumens may not be apparent to the untrained eye.  There's a lot of play for marketing purposes here.

"Perceptible equivalent" LED marketing can be even more misleading for grow lights.  For example, a "1500 watt" LED grow light currently popping up as "Amazon's Choice" only consumes 280 watts of electricity and it's suggested that it can replace a 10,000 watt HPS or Metal Halide bulb.  Even if the generous assumption was made that these LEDs were state of the art (2.6umol/J) and the HPS used for comparison was on the low end (1 umol/J), the 280 watt model would at most have a PPF equivalent to a 728 watt HPS fixture.  It's probably MUCH lower given the low price point.  The overarching message here is beware of misleading specifications and run independent tests if possible.  Look for brands that publish specifications in terms of PPF, PPFD and PPE.

LED shop lights intended for illuminating garages have begun to replace fluorescent lighting.  Generally, these products specify light output of around ~5000 lumens and power consumption of around ~50 watts.  For the brands that uphold these specs, that's ~100 lumens/watt efficiency which is on par to slightly higher with the fluorescent lights discussed previously.  I've evaluated the spectra on a few of these and the spectra tend to be very blue heavy.  They could perhaps replace a 2 bulb T8 or single bulb T5HO fixture.  However, I can't give any specific recommendations since the spectra varies and it seems manufacturers disappear as quickly as they pop up.  Many don't actually meet their published lumen specification.  And also I can't comment on the reliability of these units in wet environments.

LED tubes intended to replacement T8 fluorescent bulbs have become more common on the market as of the last few years.  Some bulbs are intended to be "plug and play" and rely on the electricity provided by the fluorescent ballast while others are direct-wire meaning that the input power (usually 120 or 277 volts) is directly wired into the tombstone connections inside the fixture which attach to the tube.  Extreme caution needs to be used to avoid electrocution when re-wiring any fixture or when installing an LED direct wire tube.

The best choice for any lighting application weighs initial cost, operational cost, lighting output, reliability, performance, thermal management as well as the many other factors involved in the application.  While shop lights and replacement bulbs prioritize fixture economy, they aren't intended to be an every day workhouse so less emphasis is given to the other factors.  Dedicated grow lights are engineered and optimized for horticultural applications.  Their heavy use demands some of the highest efficiencies of any LEDs on the market.  LED grow lights enable some possibilities not attainable with prior technologies.  Some are waterproofed in order to withstand the warm and wet environments of greenhouses and grow areas.    Other LEDs may be dimmable or have changeable spectra changes with the turn of a knob.

Greenhouse supplementation LEDs are meant to replace HPS and metal halide lighting.  They are often high powered (300 watts+) and have high efficiency.  These units are generally engineered to be compact so they aren't blocking sunlight in the greenhouse.  These units are also suitable for other high throw applications such as grow rooms and grow tents.

Large LED arrays are often used in grow rooms.  The shared power source can increase efficiency while the wide layout increases uniformity. 

Some grow lights have a sleek form factor.  These are ideal for tissue culture laboratories, seedling shelves or under bench lighting.  Hobbyists using grow racks may also find these very appealing.

Grow Light Efficiency Summary

Above is an efficiency table documenting average efficiencies of a range of technologies.  Specific brands and product lines may differ.  

It's important to remember the efficiency of the system is a product of the efficiency of each individual component.  For example, during my M.S. program, I designed and constructed some of my own own LED units using off the shelf chips.  The LED chip efficiency along with the losses from the power supply, cooling fans, and other components was required to determine the overall system efficiency. (See below about thermal concerns).

It should also be mentioned that LEDs have the longest life any lighting technology.  Generally this is specified as the LT-70 of a unit and is often measured in the tens of thousands of hours (years).

There are a number of important Thermal Considerations that must be taken into account for grow light applications.  These include HVAC costs, environmental considerations, equipment failures and loss of efficiency due to heat sinking issues.

Up next: Thermal Considerations

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