That's kind of why I wanted to post it here for discussion first. I'd rather not post anything on Github unless some research has shown it's a problem to address. The issue may simply be that the Solo's stock hardware can't handle that kind of load. In which case, it's just a matter of documentation so users don't try to do it. And making sure our parameter set for the Solo is set appropriately. But this situation in general will apply to everyone, not just the brave solos.
When 3.5 is pushed out as a production release, we're going to have a lot of this if it isn't documented well for the users. I don't believe it says anywhere not to just enable both, or that you need to change IMU masks to do it. The logical response users will have is "ohh, this is new and better, so I'm going to turn it on now!" without any other changes or considerations. Whether it's a Solo, a racer, or a heavy list octo.
So I think we need to determine what the best practices and scenarios are, document them in the Wiki, and document them briefly in the MP/Tower parameter descriptions. And make sure the release notes for 3.5 tell the user to go read that before pushing buttons.
Now that I've said all this work that "we" need to do, I supposed I should volunteer to do it. I'm not an expert on EKF. But I am happy to take on contributing to the Wiki for a lot of this documentation stuff we've been talking about lately. The solo will need a lot, and I'm happy to expand upon that for other aspects like the EKF.
@OXINARF / @Andrew_Tridgell / @rmackay9, How can I get on board for Wiki contributions?