I would point out that a beefy desktop is probably faster at compiling code than a typical cloud instance simply due to more CPU performance. So maybe up to 10-ish concurrent agents it's faster to use a local desktop than a cloud instance, and then you start to get into the territory where multiple agents are compiling code at the same time, and the cloud setup starts to win. (That's assuming the codebase takes a while to compile and pegs your CPU at 100% while doing so. If the codebase is faster to compile or uses fewer threads, then the breakeven agent count is even higher.)

Other than that, I agree with what you said. I don't know what the tradeoffs for local on-premises and cloud agents are in terms of other areas like convenience, but I do think that scalability in the cloud is a big advantage.