I think the only real potential optical issues are the adapter moving during use, axial skew and flare.
symmetric primes eg. Leica need very precise tolerances viz. skew but telecentric lenses eg. Canon probably care less. However, skew is still a very real factor. Of course, I could imagine that if the tolerances of the adapter and lens add up the wrong way you could also have infinity focus issues.
This is getting ridiculous - people who don't do engineering as a job *will* face issues when they start fitting random-sourced elements together - you can do it and try, if it works you're lucky and happy, if it *needs* to work perfectly, then you need to ask a pro dealer who knows what combinations are known to work, or take the baby to a pro repair shop and ask them for advice.
Edmund