Documentation is a separate package so unless you're careful to always install it you tend to not have it.
Busybox is definitely not 100% compatible with coreutils, lots of sloppy scripts tend to break.
For a while the default kernel was the grsec one which breaks pretty much everything. Thankfully that seems to have changed.
musl libc lacks some of the new instrumentation and bounds checking features in glibc, also the subtle differences can change how often bugs in incorrect programs get triggered. Of course musl libc is not binary compatible so closed source software won't work. (no widevine or chrome)
Some larger packages are missing, I don't know if they ever managed to package Gnome (I certainly don't blame them, it's a beast.)
OpenRC is not systemd, I prefer it though.
They have an an installation option that lets you run from tmpfs but back up /etc and automatically reinstall packages from a disk cache on reboot. This is great for that generation of macbook pro with the bad harddisk cable, mine had 8gb of ram which is plenty with a small distro like alpine as long as you're used to keeping everything in git and pushing often to branches on other machines.