Demo's on the ground are effectively useless in my opinion. Your much better spending that 90 ammo on 3 mines instead. Should be a greater investment for potentially bigger return, however it just doesn't have the effect. Will never get effects and feelings like this again

The size of the house plays a massive factor with demos. It actually usually does instant kill squads, provided it is a small/medium size building. However all the buildings that are large the garrison squad will almost always survive. It is how buildings are fucntion though, have to level the whole thing to the ground to kill stuff. I think this is wrong for demo's though, should have a higher kill rate for such an expenisve investment.
Make them longer to build and with a greater area of effect radius would be a decent idea for the current price I think.