Those are the same folks who would never consider a
DIY dual power supply mod, a multi-PSU system is perfectly fine when implemented correctly. In all honesty, it is the only way to have true rail-isolation so in that respect it is more stable than a single PSU with multiple rails.
I have heard the FUD as well, they will tell you the temps will increase and melt your system... I measured a shuttle system I used that TT 250w in and the temp increase was only 1C at most. Maybe they were talking about a system that has no exhaust at all? At least I can say I have tried it and have not experienced that particular situation. Maybe you can ask them what experience they have had w/ add-on GPU PSUs, 9 times out of 10 it will be "none". Not to mention, it sure looks cleaner than what I can do from a pair of old PSUs