I think it depends on the make. Iris finds is really, really easy to escape from some aplix nappies (Green Kids, Funky Nappies) but not so others (BG, BBOS). I think that the larger the aplix tabs, the easier they are to get out of.
Personally, I prefer SS pockets to FS (I like our FS PBPD, but love our SS ones - they're a better fit on Missy). I like our WN touchtape OS. Iris hasn't escaped from them. Yet. But that's me being super-vigilent and distracting her when she attempts to, rather than a lack of trying on her part.