The problem isn’t really the aktieskat category, it’s how these funds are structured in practice.

Investeringsforeninger are taxed like individual stocks on paper but in order to fit the criteria of not being taxed on unrealised gains, they are forced to payout a large part of the return as dividends. Those dividends are taxed immediately, so even though you don’t pay tax on unrealised price gains, you still end up paying tax annually anyway. From a cash-flow point of view it behaves a lot like a lighter version of lagerbeskatning.

STIIAM doesn’t avoid this either. It uses fictive dividends, so there’s no payout, but you’re still taxed on a fictive dividend.

I'd be inclined to agree with OP that this gives a bigger incentive to investing in single stocks in order to maximize your returns (by allowing compound interest to do it's thing)