I played against a horde ork list with my Harlequins a couple of times in events with the index army list.
With Harlequins having tons of re-rolls in melee, and four attacks each, they can actually do a lot of damage to orks. The trick is tackling one squad at a time, using the transports to hold off the other squads. If it wasn't for the games ending due to round time limit, I probably would have won both.
Too bad against Orks you're playing against the clock as much as the player.
Armies with few multi-wound models are the easiest for Harlequins to deal with, as they have many ways of dealing multiple damage. Monster Mash lists are not a problem.
The main lists that Harlequins can struggle against are vehicle heavy lists, especially with super heavies, and gun-line lists. Tau, Dakka bots, and Guilliman Gunline, reaper/ynnari are the sort of lists I fear the most.
I don't see Harlequins winning events, as the top competitive builds tend to be hard counter to pure Harlequin lists. But, they do have more tools now than the index. I'll need to play them more with the new codex to see. So far, I'm pretty happy.