Hello,
I bought my replacement battery for that same exact laptop on here. The first battery dropped very quickly in terms of performance, and as I monitored it via coconutbattery I was able to see that something was not right but I was too lazy / busy to take care of it.
Eventually I started to have random shutdown problems - most of which experienced when the MacBook Pro was running off the internal battery (not on AC). I contacted ifixit (a year after, mind you), and they sent me a new battery as a replacement.
In other words your mileage will vary, but if you go to a reputable source, they should stand behind their products. Sure, you will have to spend the time cleaning it and getting rid of the adhesive (you get better the second time around btw, hint: do not use acetone, use some light scraping with a strong plastic sprue, and use painter tape to remove the remaining residue), but in the end this is the cost of DIY.
As a side note, I should have qualified under Apple’s coating recall but the Genius flat-out refused to take the laptop when I told him it has a 3rd party battery in. So… Piece of advice, keep your old battery around just in case (or send it to me lol).