In my experience, it's partly the extreme gloss of a modern coating and partly the extreme clarity of this type of coating that makes it look odd on a vintage EM playfield.
For me, the best topcoat on an EM playfield is oil-based gloss polyurethane. The gloss is not as extreme as a modern coating, and it also has a nice amber patina that I think looks good on an EM.
- TimMe