On newer cameras, that support PDAF with adapted lenses, they work pretty well. Metering is fine. AF is pretty fast and accurate, with no need for micro-adjustment. EXIF is provided. IBIS (in body stabilization) works. Depending on the adapter IBIS and IS, for lenses that have it, may work together.
Compared to native Canon bodies, I would guess that AF probably isn't as fast, and that tracking isn't as good. On the other hand, things like manual focus magnification in the viewfinder, focus peaking, zebras, and stabilization with all lenses are things you get on the Sony bodies that you don't get on the native Canon bodies.
So, seamless? No. Pretty good, and in some ways better? Yes.
Also, some of the Sigma lenses on the MC-11 adapter are pretty much seamless.
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A7-II with kit lens and a number of legacy lenses (mostly Canon FD); NEX-7 converted to IR.