Eugene is correct. Unless you are running them right above your water without a splash shield then salt creep and corrosion should not be a big problem. There was one guy in the candlepower forums IIRC that posted a picture of an emitter he had been running over a salt water tank for a long time and it was covered in salt and corrosion but it still lit up.
Heat is really the enemy of LEDs. Like any lighting system it all depends on how it's designed and put together. If it is designed well and has adequate cooling you can expect the advertised life out of the LEDs. If it is not well cooled you can expect the LEDs to degrade faster. But LEDs themselves have been used on myriad different types of equipment in all sorts of environments so I would still argue that their reliability is well understood.
As Eugene pointed out the LEDs are best run at less than 80 degrees. The lower the better. I haven't measured the temperature of my heatsink exactly but what I can do is put my finger in one of the holes on top of the fixture and touch the heatsink. After running for several hours it feels just barely warm to the touch so it can't be above 40 degrees. I may add a temperature sensor hooked up to the Arduino when I get to that point.
As for cars, which ones seem unreliable? I have heard of some issues on Cadillacs but Hondas and Audis seem pretty reliable from my understanding. Again, Eugene is correct that the stresses on automotive components are far higher with greater temperature shifts and shocks and bumps and vibrations etc. Not generally a problem for aquariums.
So I think your criticism of LEDs could apply to T5s or MHs as well as there are good reliable systems and poorly built and unreliable systems. Doesn't matter what the technology is. Part of the problem you mention about difficulty in repairs is that in order to save on manufacturing costs the LED emitters are set up on modules in many systems and if one LED goes down the whole module has to be replaced as it is usually not simple to replace a single emitter once its on the PCB. I do give kudos to Maxspect as each of their LEDs is on a discrete star PCB and can be replaced individually if need be. Not so with the AI or several others where you have 3 to 6 emitters on one PCB. So again, depending on the design of the system, the fix could be as simple as replacing one emitter or having to replace an entire module. But replacing a module should not be more difficult or more onerous then replacing a ballast in a T5 fixture.
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