Those that have prevented Sony from releasing a competitive zoom lens so far?
These are all manufacturer constraints, not mount constraints. Regardless:
100-400 - best in class at the moment (better than Canon's in the corners, although it's a newer lens and hard to compare, while Nikon's 80-400 is way behind). I hope Canon has beaten this lens with their newest 100-500, though - it depends if the 100-500 can make f/5.6 at 400mm (with 500/7.1 being a bonus), as well as how sharp it is. There's a rumoured Canon 100-400 coming up later, but it doesn't appear to be an L lens.
200-600 - again, no-one else has anything competitive within the same class. They're either budget options with limited performance, or super-expensive. Sigma's 150-600 Sport is probably the closest comparison. The 200-600 actually holds up very well even to the Canon 200-400, except it's a stop and a third slower - weaker in the corners, but the centre holds up well, and the corners of almost any shot taken with this kind of lens will be out of focus anyway.
12-24/2.8 - just announced, samples out, seems to kick the crap out of every other <16mm UWA (zoom or prime) out there, from the limited samples available so far.
12-24/4 - matches the Canon 11-24 at half the weight and 60% of the price. Beats the other 12-24 options out there. The Sigma 14-24/2.8 DG DN is great, but doesn't reach 12mm. Nikon's 14-24 design is getting really old and is overdue for replacement.
It's just the 70-200 (second GM zoom released) , as well as the 24-70 (first GM lens ever released) that give them a bad name. To be fair, those early GM lenses (including the 85/1.4) were designed with smooth bokeh as the selling point, not sharpness - when the emphasis was changed to resolution, the lens lineup improved in a big way. And the terrible copy-to-copy variation that can lead to one lens seeming prime-sharp and the next looking like something out of 2005 - my 24-70 is very sharp, but took 4 bad copies to get to.
Those causing Sony from not releasing f1.2 primes?
Completely a manufacturer decision, not a mount limitation.
Sigma and other f/1.2 lenses adapt and work perfectly, with no more vignetting or edge clipping than on their native mounts. Even f/0.95 primes do. The designed-for-E-mount Sigma 35/1.2 is also one of the best 35mm lenses out there.
That Sony has restricted themselves to f/1.4 so far (rumoured to be changing later this year) is entirely a design and manufacturer capability decision, not one dictated by the mount.
The mount size 'constraint' theory has been pushed by Nikon ever since the size of the Z mount was announced. Oddly, it didn't seem to be a problem when F-mount SLRs were top-tier.
Any lens fast enough that it would be possible on a Z mount and not possible on E mount would be so big it would be impractical anyway.
In any case, I'm still waiting for a performance review on the R5. In terms of positioning, it seems to be a mirrorless 5D - an amalgamation of the 5D and 5Ds lines. But, capability-wise, is it an A7r3, A7r4 or A9r?