from what i can tell from playing, the smash ball breaking is based on dmg , not hits.
i can do two full dairs with mario and sometimes it dosent break. i think its "health" is randomly in the 40-60 range. (roughly)
mookie: the smash ball is harder to get than it seems, its movement is very random and effected by attacks in weird ways, and loves to be high up , alot of times above double jump distance. ive been knocked off the stage almost koed and have had others not be able to get the smash ball quick enough due to those attributes.
my only problem is with item appearance on the lowest setting, smash balls are pretty often in 4 stock matches. ive not played with 3stock at all though.. so i wonder if maybe the quickness of those would make them less rampant? ohh, and trying to stall for a smashball even in those matches was a joke.
on topic about the smashes: some of them can be used more effectively in certain ways.. imo none of them are flat out worthless. but in 1v1 matches ive survived multiple final smashes (mostly sonic, dedede's and luigis) relatively easily. if you screw up against luigis, it IS a free ko because he will just walk up and ping-upb you. sonic is just dodging, even with the player trying to just sit ontop of you you can pretty much dodge it, and if your not rocking 60%+ you can survive a few hits of him comboing you with his face. (which you can still airdodge out of i might add). Dedede's is annoying, but you can stall out of it if you know what to watch for. (you can especially ledge stall the entire **** thing if you get to it )
ive dodged marths on occasion, its pretty fast, i cant give frames or anything though. it seems with the "big hit" supers or "auto combo" supers, rolling through the intial hit is pretty on point. just requires some yomi and not mindless running away. also: lots of characters "float down" if they use their super in the air (luigi/dedede) they die if they use them off stage (luigi's might recover just soon enough if you do it high in the air.. but that would also make the super useless to begin with), ive not seen them use it in range to float and grab a ledge.
funny side note about luigis: if you try to jump out of it off the stage to grab the ledge, youll helplessly fall in tumble stun to your death if the sleep catches you in mid jump. lol... embarassing.