Obviously any PE product is better than any economy product. Economy plus on UA is just economy with more legroom (for example, an exit row seat or a bulkhead). It's not a separate class of service; you are buying an economy ticket and picking a "good" seat (either for a fee or for free if you have the appropriate status). So it's just one of the better seats in economy.

UA has a premium economy product on some routes as well. That's a separate class of service. I've flown it a few times on a couple of different long hauls (several times SFO-FRA and back; once SFO-TYO and back). It's fine; comparable to CX premium economy (I've haven't flown SQ premium economy yet so I can't compare but I assume it's about the same). You get a fairly wide recliner seat with a foot rest. The seat is wide enough that you aren't bumping your neighbor (and I'm a fairly big guy -- too wide in the shoulders for a regular economy seat) and it reclines enough to be comfortable with the foot rest. I slept well. Not as well as in Polaris, of course, but still well.

But yeah obviously if you compare premium economy (on any airline) to economy (on any airline) the premium economy product is going to win every time. That's why it's a higher class of service, because it's better.