Have a 3090, 3070 and an RX 6800. My favorite card of the lot is the RX 6800. The only real downer of the card is poor ray tracing performance when there are a lot of incoherent rays.
Other than that, it's far better than both the 3090 and 3070. Primary reasons are the following
I) Rock solid drivers this gen - haven't had a single crash ever. In comparison the 3090/3070 has had multiple CTD issues in many games
II) Way quieter and more power efficient. The 6800 is much faster than the 3070 while consuming almost identical power. You can barely hear the fans while peaking at a max 70C. The 3090 is hot and loud. Same with the 3070.
III) In standard rasterized games, there is virtually no difference in performance between the 6800 and the 3090 - none that you can detect.
IV) The driver control panel and functionality in AMD is a couple of generations ahead of nvidia. NV's driver interface is still stuck in the XP era. On top of that you have GeForce experience which feels like spyware running all the time.
Also long term, AMD supports performance updates of old cards better. When the RX580 was launched, it was at parity with the 1060 6GB. Now in modern titles, the 580 is much closer to the 1070 8GB while the 1060 struggles to match even an RX 570.