I used to haaaaaaaaaaate Frigate, but I actually kinda like it now.
The lack of an edge on the right side of the first transformation can really screw over some characters will balls recoveries. Even if it doesn't outright gimp them (which it probably won't.... most people've learned how to deal with it by now), forcing them to recover on that side will more often than not put them into a position where you can predict where they're going to go and easily punish them. Pikachu, on the other hand, doesn't have as much of a problem with this since QA is so versatile.
The stage itself is small but not super cramped, and the platforms are out of the way enough that you can still get your camp on if you need to, especially on the first transformation..... if you stand on that platform that goes up and down on the right side, when it's in its "down" position, it's the perfect place to camp and shut down any approaches.
The dip in the middle of the second transformation can mess up or help your CGs, depending on which side you're on. Generally you want to be going up the slant. Also increases your chances of being able to follow up from dtilt, fthrow, ftilt, and any other move that sends them into the slant.
I'm not exactly sure how the mechanics for the stage flip work, but as long as you're in the air, you should end up in an okay spot when the stage is done flipping. Even if it flips you under, as long as you're near one of the corners, you should be able to QA back..... most other characters would be screwed.
Somebody brought this up a while ago, but if you thunder at the exact moment the stage is flipping, thunder's hitbox gets expanded and can potentially "suck" people in. Since people are usually just concerned with making it safely through the flip, it's relatively safe to just go ahead and thunder even if you're not sure if it'll hit.
Characters I'd bring here off the top of my head:
Marth - stage dips and moving platforms can help you get past his fair camp, right side of first transformation has potential to gimp him, stage isn't small enough for him to kill too much earlier than usual
Pokemon Trainer - Ivysaur gimp potential + no major buffs to the other two
Ice Climbers - dips can mess up their CG timing, platforms to camp on, and Nana is stupid enough to gimp herself on the right side sometimes
Diddy Kong - platforms, crappy edges, and dips to potentially help vs. his glide toss shenanigans
Peach - uneven terrain is always good vs. Peach (to mess up her float spacing). her recovery sucks on the gimpy side, too.
There may be others, but I wrote this entire post completely off the top of my head. Might come back and add more later.
I'd rate it a 7/10 for Pikachu. It's a solidly good stage vs. most characters, but it doesn't give any major benefits that make it auto-win.