A combination of intense active play (combat is a clickfest) and massive walls requiring lots of meaningless waiting, with very few gameplay elements to interact with. Hostile UI and zero feedback on the effects of your purchases (combined with high prices which make bad guesses disastrous).

You can bypass virtually the entire early game if you're good at codebreaking or use a walkthrough, which is good because it got me to the first true hard stop (approximately around the time you start grinding up for the ghost) before I got invested.

The reason I call that a hard stop is that buying the gold factory triples your gold output from 1/s to 3/s and allows you to further increase it at a significant cost, but also removes the rat fighting which allows you to manually grind beyond the painfully slow trickle income. Seeing if there was any way around that, I checked the game's subreddit and apparently there's a WEEKS-LONG wall late in the game (buying an airplane for 5 million iron bars) that can only be done by leaving the game on idle forever (it doesn't even have offline progress) or cheating. The reddit also mentions a fan remake "The Gold Factory: Regilded" which rebalances prices for smoother progression. If you want to play this game, play that instead, but honestly the core game is clunky enough I'm not captivated enough to see if the rebalancing makes a difference.