I can't think of a shooting situation where a reliable camera is more important. Working with a well tested system and a backup system ready at hand is how you distinguish yourself from the amateurs. Using a non-native lens is never 100% reliable in my experience.
I could not agree more!
Yes, the Sony A9 or the A7RIII are both great bodies, however you sacrifice several key advantages with non-native lenses here!
And yes, none of the adapters are 100% reliable...
Yes, I do use Sony bodies and Canon lenses together frequently, but never for high pressure, one opportunity only shooting.
If you do want to use those bodies then get a couple of good Sony lenses to go with them...
Tony Jay