Also unfortunately it doesn’t always play nicely with lots of high energy physics software which really leans on using LD_LIBRARY_PATH and running custom executables during builds. I’ve had bad experiences with ROOT and Gaudi with no obvious paths forward for fixing things.
On `LD_LIBRARY_PATH` specifically: we intentionally inject RPATH-ing compiler wrappers into the build to avoid these types of problems, which are on by default for the ROOT and Gaudi builds. Would be curious to hear where the remaining pain points are.
For instance, several times I've had Spack in a state where it had obscure apparently internal errors that I didn't have the energy to try to diagnose, which resulted in throwing everything away and starting again. I found build recipes frequently don't actually work, especially if you're not on x86_64, though architecture accounts for a minority of the failures. You also end up quickly running out of space in ~10GB of home directory, at least if things aren't shared on the system.
There is a tension between "reproducible" static configurations and the dynamics you need, e.g. to be able to link instrumentation or acceleration libraries at run time. However, there is a Spack option to use RUNPATH instead of RPATH that I'd have to look up.