Fml, the amount of blatant Shy Guy Beach disrespect here is amazing. Probably THE most challenging technical course in any MK game ever, between the various low-trick jumps, wheelie hops over the water and surprising narrow optimal lines. Just try and get near a good player on this track.
Also hot take: Brokonut Mall is overrated, ruined by both the absurdly easy ultra and the terrible parking lot section. Ultra aside, Volcano is genuinely godlike though (but if you don't have shroom going into the second half of lap 3 you're boned lol).
Rainbow Road is mid-tier. It has like 1 cool trick and it gets some credit for being the only track where karts are are faster than bikes but that's it. Nothing special.
Anyone who puts mushroom gorge as anything but bottom-tier needs to be made to do mushroom bounces for a week without end as punishment (and then you get target shocked halfway across and have to go again.)