mnredfox

Can only really comment on STE+ here.

OLCI - Can be done via internet or your cell, but you should know Chinese for the mobile phone version. Cannot do OLCI for international flights.

Cchek-in at airport: as STEP, you can use the business lines to check in. This is actually quite a nice benefit as Chinese don't really do OLCI by habit, so the regular check in lines are quite long. Chinese also like to check all their luggage (since 1st bag is still free) which adds to the lines.

FFM: you will earn DL miles as long as the fare class is eligible on MU/CZ, check the DL website for exact fare classes.

Seating: as STEP, I've had luck accessing CZ's W cabin which is like Y+. You have to check in early though, so if you can't do OLCI you might not snag one of those seats. Often as a STEP, once boarded the FA's will come by to ask me if I need anything to let them know. Nice touch, but not super special as typically on Chinese airlines if you ask they'll help you. So shoot for Y+ (W class on CZ) as you get better food, more legroom, bottled water, and blanket.

Early boarding - sometimes done by CZ for international. Both MU/CZ have FC/J/Elite lines but they really don't work.

Lounges - you can get lounge access for international flights as per STE+ rules in CZ lounges, but not for MU lounges. Most China lounges aren't something to brag about, though some are pretty good.

UG's: Never had one, not ever expect one. China airlines won't even oversell Y and bump people into F. The rule in China pretty much seems if you want an F/J seat, go buy one.