They easy ones are quite voracious feeders and typically need larger tanks to grow the grub that they graze. The specialized feeders require very special care.
All of them require high quality water at/near ocean level parameters and not the way that most hobbyists like to keep their tanks. They are sensitive invertebrates.
While not impossible, or anything, they require a lot of work and you might have to grow food for them in separate tanks. Most people fail with them. Velvet nudibranch is a very beautiful and active creature, but will require you to grow red planaria (flatworms) in another tank for them to eat.
On top of all of this, they are short lived and only a few varieties, like Berghia, will multiply in captivity. Berghia can be fun, but they hide a lot, are not super colorful and you have to grow aiptasia in another tank for them to eat.
