Yeah but you're entirely reliant on getting those first kills and having to build an army every time you do a run.
True, it's a definite down-side, though it is pretty easily overcome with a half-decent merc in the cold plains. Once you have a couple of skeletons the rest come easy.
Plus you're missing telly without the runeword, which we all know is not cheap.
Also true, but you don't have to have teleport to be viable with a fishymancer, it just helps immensely. You could still very easily do worldstone keep runs without it and buy an enigma with all of the leet drops you get. Enigma isn't all that expensive, after all. It only requires two "high runes" to make, so you can get cheap ones for not much more than that or just make your own.
One big point FOR fishymancer, IMO, is the game creation limit you almost certainly run into with a sorc, especially once you get good and have decent gear. Sorc runs are much faster, meaning you have to create games much faster, and often end up getting temporary bans for creating too many games too quickly. Worldstone keep runs on a fishymancer on the other hand take much longer (since you literally clear WSK2, 3, and the throne/baal) so this is never an issue.
If all you care about is doing meph runs or something, then yeah a sorc should be your go-to. But a fishymancer can do literally anything, including clearing high-level areas that a Sorc isn't going to have fun trying to deal with if they even have the ability to. And in basic gear.