I would agree for me as well. 100% success unless I'm on standby or a last-minute schedule change forced me from my original itinerary onto an already full flight.

I can count at least 2 in the last year - SNA-DEN in E- was a standby and CLT-SEA that was because of a last minute itinerary change (but that was on US so it probably doesn't count).

Honestly, I wouldn't book the flight ahead of time if there aren't any E+ seats open unless there are absolutely no other options.

Heck, I wouldn't even book it if all there are left in E+ are middle seats. I'd just prefer to wait for the next flight or take an earlier one.