I have an old HP P2 Xeon machine with a bottom mounted PSU. I like it aesthetically and for the physical stability it adds, but that system was designed for it. The PSU connector is on the bottom of the board and each set of wires from the PSU is the appopriate length for where it needs to go. Unlike modern implementations, it pulls air from inside the case, which I think is more practical. It's 20 years old and still works perfectly, but it wasn't cheaply made and wasn't marketed for quietness.
If a PSU fails from heat within a reasonable lifetime, I blame it on any of these things:
1) The use of junk capacitors that will never be reliable at any temperature. I'm not going to try very hard to extend the life of a bunch of Fuhjyyus. The only real solution is to replace the caps or get a PSU that has good ones to start with (admittedly that can be hard to find).
2) The use of a cheap fan that fails. Basically the same complaint as #1 - cheap parts.
3) The use of a variable speed fan. In principle that's great, but in practice these setups seem to be skewed heavily in favor of quietness. They want NewEgg reviews to rave about how quiet the PSU is. Nobody's going to post back about long term issues from a chronically high temperature.
I don't like the idea of pulling air from under the case. This is asking for tons of dust and pet hair to get sucked in. If you keep your computer on your desk then this issue is greatly lessened. Even so, pulling air from the surface directly underneath the case is always going to be the most dust prone way to do it.
From what I've seen of modern cases, I think they tend to have filters down there nowadays. That's good, but it doesn't totally satisfy me with respect to the issue, and it's not a convenient place to need to be messing with filters.
On a conventional motherboard with top mounted power connectors, I prefer a top mounted PSU. If it's going to be in the bottom, I would prefer to pull the air from inside the case. I would also have a dust filtered intake fan in the front of the case.
But I also like external drive bays, a sealed top, and properly colored wires. Based on some recent window shopping, I think my preferences for cases are obsolete on all but the cheapest models nowadays. 😀