iFixit’s batteries do not support iPhone features such as battery health diagnostics. One side effect of this is it’s hard to tell if your phone’s performance is being throttled due to battery health.

Run times with a 3 month old iFixit battery are 40% less than apple’s stated battery life (6 hours with iFixit, vs 10 hours stated by Apple).

Apple refuses to repair iPhones with aftermarket (iFixit) parts in them. That includes batteries. They consider it a safety hazard and a liability.

So…. once you use iFixit for a battery replacement, you're banned from Apple's repair service forevermore, which means that you permanently lose functionality on your iPhone.