Any reasonable web browser lets you assign keywords to searches. No need to go through a third-party.
I've used these a lot in the past, but switched to DDG's bangs largely because it was such a pain to change browsers/devices, and none of them ever worked for me on mobile. DDG's bangs are still a little unwieldy (! isn't a very convenient character to type, especially on mobile), so an alternative would be nice. For now though, the consistency everywhere far outweighs any other advantages of the in-built browser feature.
Opera, yes.
Unfortunately the UI isn't too great -- on desktop, the suggestions between the awesomebar show you that a keyword search will happen (just like when you type something matching an open tab it tells you that it will switch to tab unless you hit the down arrow to choose something else). On mobile there is no such indication, but hitting enter after typing the keyword followed by the query still works fine and does the keyword search thing.
I might try to fix that.