You're not guaranteed SE on every boss run It's not financially prudent to apple anytime anywhere Smaller emphasis on HP washing Personal preference in terms of character skills/playstyle/difficulty Just to name a few...
I mean, you get to pew pew from a sweet mini battleship. I personally find playing sair lots of fun (speaking as a level 15x sair). I like the challenge of dismounting while bossing. Their skills are just cool in general. I mean they get a battleship, an air strike by seagulls, octopuses, flamethrowers, ice splitters, and WINGS!. Overall just a very colorful and fun class in my opinion
Sairs can generally deal more DPS at bosses that don't attack or that they can pin such as Krex, Anego, BF, etc. Sairs are a challenge when it comes to watching for boss attacks and dismounting ship, but if you get good at it, I'd say it's more advantageous than being a NL as they're not overly dependent on buffs. If you're even lucky enough to have SI and SE in the same party as a Sair the damage is amazing. Generally speaking, NL is much easier to play [more DPS due to high avoid and Shadow Shifter] and the damage is great with SE. Sair is tougher to play, but the potential is higher and they're not reliant on buffs. Both classes are fun and strong either way