Because I am exactly the kind of person to obsess about this sort of thing, when I was working on my last book, I spent a lot of time deciding how I wanted to style dashed subordinate clauses.

Personally, I think en dashes are too small and look like a mistaken use of a hyphen. I really only use them in their Chicago Manual of Style recommended uses like date ranges.

But I agree that em dashes without spaces around them look wrong. They glue the adjoining words together when the whole point is that the clause is secondary and should be set aside from the surrounding text.

I ended up using em dashes with a little blob of CSS to put a tiny amount of space on either side.