I did it once. Flaked like crazy. Never again.
Here is a blog post [0] I read a few months ago about wood finishes. I think the following applies just as much to cast iron as to the wood he is talking about:
So, when I choose a finish, I ignore the industry-standard scratch and adhesion tests. Instead, I separate finishes into two buckets:
1. Finishes that look incredible immediately but look like crap in 20 years (the short-run finishes) vs. finishes that look incredible when worn/abused (the long-run finishes).
2. Finishes that want me dead vs. finishes that I can apply while buck naked.
Basically I would equate Polyurethane with Teflon, and furniture oil with cooking oil.Teflon will be great to cook on initially. As soon as it gets damaged, it will be hard to fix without stripping and redoing everything.
Oil will take more maintenance over time, but if it starts looking bad, you just put a bit more on. That's exactly how you should treat cast iron as well.
The trouble people run into is they try to use oil to create a teflon surface. That's not what it's for.
[0] https://blog.lostartpress.com/2020/11/30/rip-the-anarchists-...