Quoted:
I personally do not like the look of them. If I'm putting on a folding front sight, I'd rather free float barrel and run a longer handguard to increase the sight radius.
I'd imagine there's some transfer of heat to the flip sight. I would wonder that running drills would generate enough heat to burn your fingers if you transitioned to irons.
Most are not pinned they use set screws and I'd image that they'd be more prone to shifting than a standard FSB.
Junk products will always be junk products, no argument there. Loos are subjective, no argument there either. I personally would rather drop a longer handguard on myself as well.
Of course anything attached to the gas block is going to get hot, but if you are being shot at and have optic failure, the least of your worries is the sight getting hot. If you are just running drills, it shouldn't be an issue. I've had a real world optic failure, so I don't say that lightly.
As far as shifting, check out the youtube video or go to the Vltor webpage to watch a vid on how secure GOOD ones are, plus you can always pin them in addition if desired.
Cheaper ones like the YHM I avoid personally, I've seen too many of them loosen up and even come apart on LEO weapons, specifically on Bushmasters.