No problems with infinity focus with T mount lenses. The T mount was designed specifically to let you use the same T mount lens with different camera brands by using the appropriate T Ring for the camera - the depth of the T ring compensates for the different camera flange to film/sensor distances.
Big drawback with the T mount is that there's no mechanical or electronic aperture coupling available - which means that a lot of entry level DSLRs won't meter with the lens at all (higher end Nikons that didn't need the electronic coupling worked OK; not sure what the situation is for Canon bodies). Also, native Canon lenses can have a slightly bigger clear aperture than the T mount diameter - so the native lenses can go to slightly faster max aperture values than a T mount equivalent, but that's not usually an issue for the sort of lenses that come in T mount fittings these days.
I've used five different mirror lenses over the years:
-An old Meade 1000mm f11 camera lens/spotting scope (basically, this was a camera lens focusing style mak scope that worked either as a camera lens (with an extension tube to a T ring) or a telescope (with a star diagonal).
-An old Tamron 500mm / f8 mirror lens with an adaptall mount (similar idea to the T mount, but extended so that (with other lenses) you had basic mechanical aperture coupling. The mirror lens was fixed at f8, though
Of those, the AF on the Minolta lens worked fine - I'm surprised there aren't more AF mirrors around.
The Tamron and the newer Nikon also made nice butterfly lenses - they both focused close enough that you could get a butterfly to a decent size in the frame from far enough away that you didn't frighten it off. The older Nikon had a much longer minimum focus distance, which is why I eventually switched to the newer version. Quality on the Nikons was higher than the Tamron, but the Tamron was pretty good - and packed down smaller than the Nikon (The lens hood reversed around the lens for transport, which gave you a shorter body).
The 1000mm Meade lens worked pretty well, on a tripod for wildlife shots or handheld in bright lighting for things like motor racing shots. But it benefitted from improvising a lens hood, since the front element was basically at the front of the tube. Usable without, but contrast improved with the hood.
These days, I usually use my 80-400mm Nikon image stabilised zoom - it's not quite as long as the 500mm mirrors were, but the AF and VR capability make up for that, and the close focus at 400mm also makes for a nice butterfly/ large insect lens. Or if it's worth the hassle of carting a heavy manual focus setup along, then I can use my 106mm astrograph refractor at 530mm f5 or 840mm f8, which gives extremely high quality results. But by the time you add a suitable tripod and mount, it's bulky and heavy enough that it's really restricted to specially planned trips - not something you'd normally walk around with.