1. There is some feeling that the EF 24-70 mk2 isn't a good lens?...must say I'm a bit surprised at that, it seemed ok to me
no it is not. I strongly regret taking it to some of the nicest places on my 22MP 5D3 and dealing with crippled edges is now next to impossible. It is back and forth trip to sharpen AI and it is still not that great. On 50MP as you guessed it proved to be a complete disaster; I only regret not selling it on years ago.
The big problem is that once your centre is focused on something reasonable some distance away, your edges will be focus only a few short meters in. Great if you are shooting a street scene into a distance - your focus will be perfect at much lower aperture than you expect. All flat views have no chance even at f/11. Likewise, forget group shots in a line. Lens confirmed to be a good copy by Canon btw.
35-50mm is in fact particularly tragic.
Not a major problem if you only shoot isolated subjects with shallow DOF, however it is still lacking microcontrast compared to most other lenses. The good part is flare resistance and nice colours. On 12MP cameras that probably would have been enough.
Any Sigma ART prime will eat it for breakfast at any aperture, and likewise would do even a cheapo Tamron 28-75mm III G2 (not available for Canon ONLY). Yes, if you go for a different brand you can have a better brand new lens than this outdated thing used, and probably heavily abused.
Again I feel strongly about this abomination of lens, because it left me to pick up the pieces of images that could have been much sharper and easier to print.
When it comes to RF equivalent, yes it will be better by a little, cost a fortune more and will still fall short of the expectations. Both MTF graphs and comparisons on the digital picture seem to confirm this. All of this when you can buy like 3-4 primes, spend just around £1k and resolve 50MP easily.