Personally, I felt this way about SC2 as compared to SC1.
I think one of the largest changes I noticed was an increase in the hardness of counters as well as an increase in the APM necessary to properly wield certain units.
For the first point, in SC1, if I had suboptimal army composition against my opponent, I would likely lose any battles that ensued unless my tactics were quite superior- but I'd likely due some damage in the process, at least. In contrast, in SC2, it always felt like a failure to properly anticipate army makeup would frequently lead to completely lopsided battles where my force might be completely annihilated without doing measurable damage to the opponent.
This was aggravated by the second point. I'm not a professional player and my APM has never been amazing, and I'm fine with that. But the more complex required army composition put a higher emphasis on APM and in-battle tactics, especially with heterogenous forces and units, like blink stalkers, that really benefit from very high levels of micro. I'll be the first to admit that, when watching replays, the things that skilled players could pull off in SC2 was pretty amazing to see- but that didn't really help my playing experience.
There result was a game that felt significantly more frustrating than SC1, even if my win/loss ratio wasn't actually all that different.