I don't think there is a problem having a OP villain be it metahumans/magicians.
The biggest problem with DD was that he was around for to long while being OP .
Having Ollie useless vs him for whole season until last 2 ep was a problem .
With Slade they did it right by having come in half way into the season .
There's another key difference, though. With Slade, the Mirakuru made him unbeatable, so they plotted out a way to take away that advantage and just turn him into a tough opponent. With Darhk and Reiter, the idol made them unbeatable until they were just beatable, which was stupid.
If you take away the magic items, here's another example of it being done wrong: Ra's. In his first fight against Oliver, Ollie couldn't even touch him while he held back. Then suddenly, by the end of the season, they were on equal footing, with Ollie being a little better? That's nonsense, and the victory feels super hollow because he didn't earn it. It would have been equally--maybe more--satisfying had Barry just shown up and fucked him up with Speedforce. And they did the same shit with Zoom and Vandal Savage: they were flat out leagues ahead of everyone else in their first appearances, and then suddenly they weren't. Savage had it the worst, where in the beginning he easily curb stomped Green Arrow
and Flash, and by the end of LoT season 1, he was getting bodied, solo, by Sara.
Again, this is why Merlyn was a good big bad in season 1. He was clearly better than Oliver, but he held back in their last fight because he thought of Ollie as a surrogate son,
but he still won the fight. He only lost in the end because he got stabbed after he thought the fight was already over, and by season 2, we learned that that was his plan all along. That's what I'm hoping Prometheus will be like.
I actually really like that he's been making his kills off screen for now, so we can't even begin to gauge his abilities.