What can be done in workflow but can't be done in SuiteScripts?

you can control it at the functional level vs getting into script changes

But using workflow is not recommended due to speed, so I'd like to know when I should use workflow

Considering we have a workflow action script I don't think there is anything that's not possible in the script that is possible in the workflow. On the other hand you can do major/complex things in scripts.

Lock a record

I find that things built through a workflow are significantly less susceptible to being broken by new releases, so for accounts where there's not as much concern about performance I will build workflows. Obviously there are a ton of scenarios where you can only do it through scripting, but honestly scripting is a last resort for me unless that customer has developer resources on staff, because it is waaay easier for them to maintain workflows and it is much less likely to break in the first place.

Thank you all