While you can use pip to install a package in a conda Python environment, such packages do not appear in the ArcGIS Pro Python Package Manager, leaving an un-savvy user with a misleading understanding of which Python modules are actually installed in a given environment. However, the 'conda list' command will display all installed modules in a conda environment, including those installed using pip. It would be very nice if the Python Package Manager would at least display packages installed with pip, like the 'conda list' command. (With the understanding that package metadata other than version would be unavailable for the pip-installed packages, since pip packages do no include conda metadata.) For extra credit, it would be really great if the Python Module Manager were able to search for and install pip packages.
Justification: Many Python modules are available for installation via pip rather than conda. In many cases where both conda and pip packages are available for a given Python module, the conda package will be several versions behind the pip version. If you need the latest and greatest for a given Python module, often only a pip package will be available. The reason for this is that pip is the primary installation tool for pure Python modules in the larger Python community. (Conda, on the other hand, offers built-in environment management, and can handle distribution of non-Python dependencies, which pip cannot. So while it's not as ubiquitous as pip, conda is a technically superior tool for managing Python installations, and a good choice for Python module/environment management in ArcGIS Pro.)