the arm very rarely falls out on my Mascis, it spins around maybe a little more freely than i'd like and if i do jam it right in it comes loose quick enough, but it's never been a huge issue to me, always worked fine at gigs.
the only thing that pissed me off about the Mascis was the bridge, the whole point of the AOM and sharper break angle was to stop strings popping out and i would still regularly pop strings

i have a mastery vibrato on another guitar which is amazingly solid and has adjustable stiffness but i don't think it's the sort of thing worth paying for to put on a squier for the moderate difference it makes.
and i have mixed feelings about the mastery bridge too, i sort of want to compare it to a staytrem because i find the saddles a tiny bit uncomfortable to rest on and they seem to budge from side to side a little, which weirds me out.