As long as your LED fixture is protected from splashing and moisture, there shouldn't be any moisture issues. As for longevity, properly cooled LED's will last a long time. I always have glass tops on my tanks so my lights aren't exposed to water/moisture.
I bought a commercial product a year and half ago, a Vertex Illumilux LED strip, and had used it for about 7 months before going DIY. It worked just fine even though it felt really hot, but the manufacturer said it was acceptable for that fixture. It still works but is in a box now since I made a fixture myself and don't have a need for the extra light. In DIY builds, it is easy to replace any LED's if they burn out. Commercial fixtures are a bit more complicated to replace the LED's if they burn out unless you know how to reflow solder. So far my builds have had no problems.