I've just recently finished a Steel Monotype run (on set with no items in battle) and it was really fun in my opinion! Definitely recommend it. I chose steel because it's one of my favourite types, and Mawile is like my favourite Pokemon (even though I didn't get to use its Mega).
There's quite a steady progression of Pokémon available to you, but most of the early ones are quite weak and forces you to use. Wormadam is really strong early thanks to Quiver Dance, especially with a beneficial Hidden Power type. By the end you can have some real monsters on your team, though. Like Excadrill, Lucario, Aegislash, Magnezone, etc.
If you're gonna do a Steel run and need any advice, don't hesitate to ask. I've recorded (almost) all of my boss battles if you'd like to see them. My biggest struggle early game was, surprisingly, Shade and a couple of Fern fights. Later on Charlotte, Terra, and Amaria posed a lot of trouble for me personally.