Hi
In terms of internal design, not much different.
the question should be revised, what could go wrong if Wafer and Lug type are selected.
In oil and gas industry, and probably most industries try to imagine worst case scenario. What will happened in case pool of fire occurs? or what can go worng when valve about to be dismantled from the line? etc.
Wafer: a compact valve squeeze between two (Pipe) flanges with (normally) exposed bolts
due to bolts elongated in the case of pool of fire
Lug: More or less same like Wafer type however the bolts covered by the body.
some design have tapped / threaded through hole. Imagine big size, under ground and bolts are corroded. it would take sometime to dismantle the bolts in places hard to reach by hands/cutting machine.
Unlike double flange, normally only one bolt used, there is also some risk attached to it.
I would say if the service inside the valve is not fire proof and prone for contributing larger fire, then aside from Water and Nitrogen services not suggested to use Wafer and sometime Lug type.
Regards,
MR
All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected