I've never had a cake fall with SPS. If the cake tiers weren't level, though, it could get wonky looking, and part of a tier could get squished if there was cake supporting cake. Even with a great support system, tiers still have to be level and the SPS plate has to be *just* on top of the tier - never below the top of the icing.
That said, while I'm not a fan of fresh flowers on my cakes, I ALWAYS put them on myself. Or let the one florist in town who I trusted help me if it was a lot of flowers. My contract had a long warning clause about using fresh flowers on a cake - which brides had to initial separately, that put all the risk on them and the florist for choosing appropriate flowers.
I've witnessed florists poking flowers into cakes willy nilly, pulling them out if they didn't look exactly right and poking them in again. The poor cake was Swiss cheesed.