My entire vault was wiped out after a radroach invasion was unable to be killed, just to prove my point I had 1 rad Roach vs 6 armed people with armor and the radroach killed them all and killed my 3 Mr handys
My recommendation is to only upgrade romms once to level 2....never upgrade anything to lv3 unless everyone in every room has 16+ damage weapons and are over level 40 for hit points