I recently tried out BE on a recent flight. There was about a $150 difference between BE and regular economy fare. I decided to pay extra for MCE at T-48h, which was ~$35.

I was able to check-in online and print out a BP, which looked like a regular BP with my EXP status, boarding group 2, etc. I didn't have anything written on it about BE or the "B" fare class.

At the gate, the GA was calling all the BE passengers by last name to verify that they did not have a roll-aboard bag with them. I heard my name and approached the desk, but the GA looked at my BP and said they were only checking passengers with Group 9 and since I was in Group 2, I was fine.

I boarded normally with Group 2 and sat in my MCE seat. During the flight, FAs did not offer any complementary alcoholic beverage or snacks for an EXP, but then I didn't inquire either.

If I weren't too concerned about the EQM/RDM and there was a large enough price difference, I think BE is not the worst for elites and certain credit card holders, as long as your expectation is realistic. I have paid extra for an extra leg room seat on other airlines (including Delta and a few low-cost airlines), so I don't think the seat selection fee is a deal-breaker.

I haven't tried Basic Economy on United Airline yet. However I believe UA elites also receive priority boarding and bag allowance, but they do not receive any Premier qualifying miles or Premier Qualifying dollars for BE flights. It seems UA BE does not have the ability to pay extra at T-48h for an Economy Plus seating.