Hes very difficult to learn and even more difficult to master. Most people just arent good with him yet. A lot of mage players arent good with his type of playstyle. Jungle darcy is significantly stronger than mid darcy.

Just like when elsu came out and no one played him well, it will be sometime before people realize how insane darcy jg is. Darcy jg changes a lot and it will take awhile before most understand all the nuances. I would speculate hes nearly as strong at low elos or anything below diamond for that matter. In masters though a good darcy jg is a nightmare.

He brings CC, survivability, mobility, and sustain/burst magic damage. Having a jg provide this cannot be understated. In addition to the mid mage ability damage and physical damage of the adc, this effectively makes the enemies armor items a lot less useful. If they build normal defensive builds, the mages crush, if they stack magic D the ADC crushes. Zill is the only jg to bring big ability power damage but hes melee and can only really threaten squishies who dont build armor. Having an ability power ranged sustain burst cc jg changes everything.

Similar to how lindis (only really great mm jg) in addition to a lane adc is incredibly promlematic for tanks, darcy dramatically changes how your team AS A WHOLE deals damage. His ult is also great vs tanks. It can disengage them, snag and kill them when they are zoning, avoid them or escape, and even bypass them to get to a low hp backliner. Its all about the team. Cant look at him in a vacuum. Its all relative.

If enough players get good at jungle darcy and the meta shifts, tanks will become less useful and engage tanks will become more useful than peel tanks or supports. This is because darcy jg meta would be poke. Poke > fortress(counter engage) > engage > poke.