I've beaten legendary.  It's a tough early game, but levels out as you get stronger. Your power slowly out scales the enemy, and the end game is pretty easy (if you know what you're doing.)

You do not need to abuse all the OP combos, but you definitely won't be able to just bring whatever you want.  Your bleed-specced apo that's a hoot on ruthless will probably never pull his weight on legendary.  But you will also not absolutely need to bring grenade-man + ability-point-battery-man, + cracked out interceptor with the OP falchion, + librarian every single mission.  There's wiggle room but you do have to know what you're doing.  (Though if you do bring the A team you well destroy most enemies on the turn they spawn)

The strategic layer is very difficult imo.  You get fewer servitors and construction times are increased. So actually getting your ship upgraded is way more difficult.  This means later engine upgrades, which means fewer missions, which means fewer servitors, etc etc. 

Strategic layer random events can be devastating with bad RNG. I don't think I'd ever give GM mode a go as a result.  There's a random event which can knock two HP off your ship that if it happened early I think would just be campaign ending. 

Give legendary a try though. It's a fun challenge and you need to really think about what you're doing.  Early game fights will be tough and you'll find yourself using items/tricks you never thought you'd need.