After running into some potential design issues with a pool re-plumbing project, it occurred to me that using unions on a number of key connections (rather than gluing directly) would really help solve some problems, both with the installation itself and with various issues that could come up in the future. I really don't see many unions, however, when I look at other people's equipment decks. I sometimes see unions on the pump connections, but that's about it. Are unions problematic when used on pool plumbing -- i.e. are there issues with leaking, for example? I would think that quality unions would work fine indefinitely, and would be relatively easy to fix should they ever develop problems (i.e. lube or replace o-rings.) There are cost considerations, but it seems like the extra cost would be far outweighed by the advantages of making installation easier, and making any future fixes VASTLY easier. Other than cost and a bit of extra time, are there any reasons not to use unions for all the major connection points?
On a related note, are there any specific types or brands of unions that are particularly good for pool plumbing?