So, turns out, if you know the Legion assassins are coming, they're pretty vulnerable to the grenade rifle/launcher while running a group. (If you don't have the pre-order packs, scrounge around Nipton.)

One of the best things about Bethesda games in general is being able to spice them to individual tastes. I love the Dwemer Autocannon's personally, and that you have to be quite the smith to make them is totally reasonable to me. You don't like them, and can thus pick a different perk mod that leaves them out. (Also the auto-cannon's are finicky to use, require you remain near them and only fire where your cursor is pointing. Plus the projectiles are slow as all get out, so dragons the one thing you'd want to use them against, they can't hit most of the time.)