> - My ISP gives me a /64, what am I supposed to do with that anyways?

For me, it is main problem. /64 is too small: SLAAC needs /64 per collision domain, and I have more than one (wired network, my WiFi, guest WiFi, control plane for UniFI APs), and it is painful to distribute /64 among them. I'm using HE tunnel which provides /48 to client and it is easy to configure, as intended.

There is recommendation (SHOULD, not MUST in RFC lingo) for ISPs to provide at least /56 to clients, but most domestic ISPs ignore this recommendation.

  > - What happens if my ISP decides to change my prefix ?

And it is another problem: tooling. There is no standard way to reconfigure router with dynamic prefix(es). Yes, it is possible to write scripts for it, but it will be fragile. No Linux distribution or FreeBSD is ready to have dynamically allocated prefixes. It is not a real problem with IPv4 because real life practice to dynamically allocate one address and then configuration changes are trivial, and if you are delegated /24, it is typically static delegation.