In reality you just end up making a shitty, nerfed version of a programming language, that business users can't understand, because you still have to understand conditional logic to model workflows, oh and your documentation is terrible because devs don't bother with the boring stuff. Most of the time the devs end up implementing the workflows anyway because they don't actually work properly.
If you really need a workflow engine definitely use something off-the-shelf, but I would go so far as to say that in the 95% case, you don't even need a workflow engine: you need a developer who is capable of writing some python scripts. Even if you pay a developer a full salary to do nothing but sit around and make changes to Python scripts on-demand, that's still going to be way cheaper than the complicated workflow engine solution, which will probably require a team (or multiple teams) to maintain.