I linked a post somewhere else from Jason Cammisa talking about how specifically the wagon is extremely poorly sprung and handles horrendously. This was disappointing for me because I was considering it as I felt the design suits a wagon a bit better.
The weight (as I mentioned before as well) is impossible to ignore EVEN on the road. It is a very fast car but not a very good car as a replacement for the F90 and certainly doesn't feel like an F90 upgrade.
I do believe my F90 'knows' that I am shopping for cars and every time I have driven one and hopped back into my F90 it has made sure to impress me

M4 Cabrio, AMG GTC and a few Porsches & Ferraris - then you hop into the dad car and apart from being fast/immediate it even feels more involving in most cases.
How it feels aside, whenever I have seen G90s out and about I have never felt like I needed to trade my F90 or that I was missing out (which was the case sometimes when I had my F10 and the F90 was new).
Wagon is a novelty as well I guess and I am perhaps trying too to find a reason to like the new M5 in wagon or sedan form. Even with all that trying, I just can't see it.