The kernel parts of MOFED are largely backports of the latest and greatest upstream kernel drivers to the various distro kernels their customers actually run. (The non-kernel parts of MOFED is mostly open source but does contain some proprietary special sauce on top, like IIRC SHARP support isn't available in FOSS.). The HPC community does tend to want to use the latest RDMA drivers as those are critical for at scale performance.
For Lustre, the client driver was upstreamed into staging, where it sat AFAIU largely unused for a few years until it was ripped out again. The problem was that Lustre developers didn't adopt an upstream-first development approach, and thus the in-kernel driver was basically a throw over the fence fork that nobody cared about. I think there is an effort to try again and hopefully adopt an upstream-first approach, remains to be seen whether it'll succeed.
- Lustre releases target distro kernels, upstream would likely break.
- Distro stays on top of CVE's etc. and provide updates when needed.
- HW likely certified for a few supported distros only, use anything else and you're on your own.