As NT1440 mentioned, Apple techs are not allowed to proceed if the one of the battery pull tabs snaps. This is a safety precaution because prying the battery out introduces a risk of puncture. Safety of the technicians is more important than the price of replacing an iPhone.
From experience, one of the most common reasons for the tabs to snap is the black plastic wrapper on the bottom edge of the battery sticking to the pull tab on its way out. The wrapper protects the delicate battery cable and other external circuitry. That sticky wrapper often gets peeled slightly by the (white) adhesive tab as it passes over it. The underside of the black wrapper also has adhesive on it, so it gets super stuck to the white adhesive and often causes the latter to tear. Once that happens, it's often game over. It's a lottery as to whether or not this happens because you can't see it (it's on the underside of the battery) and it can happen regardless of how slowly and carefully you remove the white adhesive.
Especially given the huge number of these battery replacements Apple are having to do, it's really not surprising that these tabs break on a regular basis. Every time that happens, they replace the whole unit.
From experience, one of the most common reasons for the tabs to snap is the black plastic wrapper on the bottom edge of the battery sticking to the pull tab on its way out. The wrapper protects the delicate battery cable and other external circuitry. That sticky wrapper often gets peeled slightly by the (white) adhesive tab as it passes over it. The underside of the black wrapper also has adhesive on it, so it gets super stuck to the white adhesive and often causes the latter to tear. Once that happens, it's often game over. It's a lottery as to whether or not this happens because you can't see it (it's on the underside of the battery) and it can happen regardless of how slowly and carefully you remove the white adhesive.
Especially given the huge number of these battery replacements Apple are having to do, it's really not surprising that these tabs break on a regular basis. Every time that happens, they replace the whole unit.