The liners hold up great! There aren't seems that they could split at or anything, so you're safe.
FYI: Playtex has recently switched to 10-ounce "expandable" liners. All of the old bottles only have 8-ounce-liner markings on them and the new ones (that I've come across) have marking for both types of liners. You can no longer find 8-ounce Playtex-brand liners, but Target makes some that I think work just as well. (I did buy a grocery store brand that I thought was awful - not sure if I can say what kind.) There are also markings on the liners, but I find them hard to read, so I've been using the Target liners with all my old bottles from DS for my DD. For making formula, I would think the 10-ounce would be preferable, since you can put the 8 ounces of water + the powder in and have room to mix.