I don't think they're banned. Winter warz is a canadian player that uses a keyboard. I've seen him use it at just about every event he's at, which includes majors.
Skip to the end, around 5:01, you can see he's got the keyboard
As far as what I think, I use a hitbox and I really enjoy it. Some commands are easier and other are harder opposed to a controller, at least for me. I used to play a lot of MAME games like MvC and I'd map/test the game out with my keyboard to see if the game ran ok but I ended up liking it and using the keyboard more often than a controller until I got into using a fightstick. They are essentially the same principle but if you want my opinion, a hitbox is a much better option unless you have some odd click/understanding with a keyboard. Hitboxes are made for fighting games, and they have all the traits of a stick just with buttons instead of the stick. You could use a keyboard as a preliminary test to see if you'll like it but you won't really know until you actually play around with a hitbox for a while. It helps if you've used a stick before.
And they're not $400, more like $200. I think I paid $150 for mine. You can use the PS3 one if you have a PS4 but only for MKX and Skullgirls.