I like the idea of immutable linux distros and would love to use one - but it seems like you will run into tons of issues with apps? no? Anyone has first hand experience?

I have been using these setups since they became available. More or less.

They work very well. Like with everything Linux the more you are aware of how they work and what they do the better off you are when you are using them.

For most desktop applications Flatpak are a win. Distributions really don't have the sources to QA and troubleshoot every single application they share with users. Some, like Ubuntu, have policies on support that might suprise some people (like Universe and Multiverse see no security updates/support besides "community support" if you are using LTS releases)

Distributions do a very good job on a handful of applications and libraries. High profile stuff. But for the majority of things out there it really is up to users to do the testing and reporting. It is the end users doing most of the QA'ng.

Also Flatpak allows you to use arbitrary versions of applications, although that does require some command line use.

So for Flatpak apps that have upstream support they often are better quality then what you'd get out of a typical distribution. I had that experience with some non-Steam games and emulators in Arch. The flatpak versions just were nicer overall.

Of course that isn't true for all flatpak applications.

For things, like setting up development environments and compiler tooling for writing software... or doing a lot of extra stuff with Steam games (like adding on VR support, etc) you can just use distrobox or toolbx.

Of course that is a lot more work. But it is the same sort of work you'd need to do if you were not using Flatpak in the first place.