I feel like most of the new stages they added are iffy depending on the mode (save Hammerhead, which is all horrible all the time). The only stage that was added in an update that I consistently enjoy is Camp Triggerfish. Piranha Pit is one of my favorite stages to play RM in, but it's awkward for TC & especially SZ. I totally agree Port Mackerel is only situationally fun. Personally I like TC on that stage, but SZ can be so terrible there...ridiculous amount of huge awkward stage obstacles, covering turf properly is too obnoxious.
I also second the fact that Museum d'Alfonsino seemed okay at first, but I don't always enjoy it anymore. But for me, SZ has been the awkward mode for that stage. In general anything with a big giant obstruction right in the center of the Splat Zone (like in Port Mackerel) makes for really wonky, frustrating matches. I feel like it's sometimes disproportionate in determining who's in (or loses) control. Then again, since the new modes were introduced, I've come to like Splat Zones less and less in general. It's the least fun ranked mode at this point imo. Slower paced, harder to make comebacks. Again, though, it really depends on the stage.