Now that I've seen an area that apparently goes on forever (yes, there's an endless part of this game), I feel like I've seen everything in this game. I was hoping to find out what black orbs are first, but I can't tell if they're even in the game yet, given that updates still happen and that there's a guy who got past level 500 that hasn't found one (you can find the post in the Zombidle subreddit). I have other things to do, so I have no intention playing this much more, if any more.
That said, I did play this for quite some time, and can absolutely say this is much better than clicker heroes. The music never feels repetitive, you don't have to spend white orbs to get items, and the occasional boosts are fun to get too. The ad watching thing I don't mind since this is an idle game, these games involve tons of waiting in the first place. The only real problem is that the bosses after the stone golem are pathetic. You'd think they'd be as much of a challenge as the first boss, but it's disappointing that their health is dozens of levels behind their displayed difficulty.