I can only speak to Spack in particular, but the main issue that I found with it was balancing researcher expectations for package installation speed with compile times. For most packages, compile times aren't a huge problem, but compilers themselves can take days to build, and it isn't unheard of for researchers to want a recent version of gcc for some of their environments.
In theory this isn't an issue with Spack (assuming that you have a largely homogeneous set of hardware or don't use CPU family-specific instruction sets), since you can set up cached, pre-compiled binaries on a mirror server (similar to a yum repo) and have people install from there.
Spack, however, has a lot of power/complexity. A lot of untamed power that means that bugs can sometimes be more likely than in other, more mature (or mature-ish) package managers. Namely, Spack allows you to not only specify the version number of a package, but also the compiler that you use to make that package, specific versions of dependencies that you want to use, which implementation of an API you want to use (i.e., MPICH or OpenMPI for MPI), and compiler flags for that package. When you run an install command / specify what you want to install, Spack then performs dependency resolution and "concretizes" a DAG that fulfills all of the constraints.
The issue that I ran into was that if you don't specify everything, Spack makes decisions for you about which version of a dependency, which compiler, etc to use (i.e., it fills in free variables in a space with a lot of dimensions). This would be great and dandy normally, although the version of Spack that I used occasionally constructed totally different graphs for the same "spack install gcc" command (if I recall correctly; take all of this with a grain of salt b/c I might be misremembering). This meant that it wouldn't use cached versions of gcc that had already been built, and ended up rebuilding minor variants of gcc with options I didn't care about.
At National Labs and larger outfits, the trade-offs between this kind of complexity/power and the accompanying bugginess (Spack has yet to hit 1.0) seem to favor complexity/power while accepting these sorts of bugs, but I don't work at a larger outfit and my group didn't need that level of power/control over dependencies and rather needed something that "just worked" and would allow researchers to be able to install packages independently of us (IT people). conda (mostly) fit the bill for this. I still think that Spack is the future and it has a special place in my heart, but it will have to be more stable for me to want to use it in production.