One, you could work in actual PowerShell, instead of PowerShell-but-not-really. The runspaces are simply PowerShell instances, running PowerShell code. Workflow isn’t PowerShell code, and it isn’t run by PowerShell - which is why you run into confusing bits. Runspaces would likely run more efficiently; WWF has a lot more overhead and is designed for long-running tasks that need to survive interruption, which is why that overhead exists. If your goal is to simply run a lot of things in parallel, Workflow isn’t actually the first tool that would come to mind. Yes, it can do that, but it isn’t its main strength, and it isn’t its headline feature.
The Runspace module is, as Peter said, just a way to make using runspaces easier. What you essentially do in .NET is spin up new threads of execution (and that’s where the throttling happens), tell each thread to spin up a PowerShell runspace, and then execute whatever code you have. You get however many threads running in parallel, and you aggregate whatever results you want. It might take some learning on your part, but it’s going to be substantially less learning than Workflow. Those threads all run a real PowerShell script, just as if you were running it in the console yourself. They use the same credential you’d use. They don’t have nearly the complexity of Workflow, and you wouldn’t need a whole separate set of documentation. It’s basically just multi-threading PowerShell; it’s not even all that unlike jobs, although jobs apply some overhead and partitioning that you wouldn’t have if you just worked directly with runspaces.
For me, the correct question is, “why would I move to Workflow instead of runspaces?” Workflow is like a 50mm cannon. Before I go for the cannon, I want to see if a 9mm handgun wild the job instead. I wouldn’t ask, “what are the advantages of a 9mm over a 50mm,” I’d ask “what about a 9mm won’t get the job done, thereby forcing me to a 50mm?” For parallelization, it’s jobs first (because they’re easy), runspaces next (a bit harder, but not much), then workflow (much harder and more complex).