Anecdotes are worthless, but in the past half decade the only people I know who have successfully replaced a smartphone or pad battery have been Apple device owners. Apple has an easy, clear, successful program for replacing batteries, and most find the pricing entirely reasonable.
A friend did buy a Nexus 6P battery replacement kit from Amazon, but the process was so byzantine, and no shop was willing to do it, that they just ended up getting a new device.
I did like back in the day when I could pop the back off my Samsung Galaxy and replace the battery, but those days are gone and Apple is the only company that has any sort of concerted program to keep their older devices operating.