I had the "opportunity" to fly MU (China Eastern) and DL a few weeks ago in paid Business Class and wanted to share my experiences for those who might be considering accruing mileage with this "Tier 1" Delta partner.

Reservation / Seat Selection
MU: No issues with reservation. Used DL app to select seat, which upon calling MU, they apparently saw, but this caused issues with their system in "undoing" the seat selection. It was very confusing and took a lot of time on hold, but eventually the seat selection by MU held.
DL: No issues on DL metal.

Using Points to Upgrade to 1st class
MU: Technically one can upgrade a Business Class ticket to their First Class product with MU miles (no SkyTeam upgrade available on MU). After calling in multiple times, I can assure you that if you don't speak Mandarin, you don't want to attempt this.
DL: N/A (no 3-cabin 1st class)

Airport Experience
MU: No issues transitioning from DL domestic to MU international via the new airside tunnel to LAX's TBIT. No additional security and they were able to print out my MU boarding pass at the gate (bags were checked by DL earlier in the day). Not possible to use miles to upgrade at the gate, but they'll happily take $3000+ to go from J to F.
DL: Bag was tagged all the way through on both DL and MU segments without issue (single PNR)

On board experience
MU (New 777-300ER LAX-PVG and PVG-JFK):

• Juice offered as PDB. Meal service was odd - they brought nuts and a hot towel immediately, but then came by with drinks about 30 minutes later. That was the 1 and only time drinks were offered, with the exception of before landing where they came by 1 more time with breakfast. Food was passable on the outbound, inedible on the return. They ran out of my first choice both flights (was seated mid cabin). Menu was odd - they wouldn't let me keep it - they just handed it out, waited 30 seconds for my selection, and took back the menu. It was printed with menus for various times of year with tabs in the booklet. Another oddity: no dessert was offered on the LAX-PVG flight because it was a "night time flight" (a note printed in the menu indicated this point). On the return, they came by with random items in plastic wrap - some had cheese in it (inedible) and some had small desserts (inedible).
• IFE selection and screen size was great. However, every time they made an announcement (which would pause the IFE system), it would exit out of the movie you were watching, not save the spot, and you'd have to start from scratch and fast forward. Imagine this happening multiple times per flight - it was absurd. Was not an anomaly with 1 segment as it was the same thing on the way home and happened to all passengers.
• FAs were nice, but spoke very little English. Service was limited in general but they would accommodate requests if you asked. Cabin was new, but filthy (not properly cleaned between flights).
• Seats were lie flat and wider than DL, but hard as a rock. I needed to use a 2nd blanket as a cushion. Overall I slept well despite the hot cabin.

DL (A330-300 and 767-400):

• In general meals and wine were far better on DL, except on 1 leg where the food was inedible (I tried 2 choices). Usually DL is consistently above average but not great in this regard. Hard product is similar to MU on the DL A330 but I'd say MU has an edge compared to the DL 764.

Baggage Handling
I had the chance to deal with an baggage snafu that was entirely caused by DL not loading my bag on the domestic leg of my trip. At least I could use the DL App to track the situation. They re-routed the bag which arrived PVG 48 hours late and traveled exclusively on DL metal (even though I was on DL/MU)
MU: Their airport filing system in PVG was acceptable, though 100% paper based and manual. Calling for updates was also acceptable during the limited hours of operation, and they spoke sufficient English. I spent about $200 worth of clothing for a business meeting. MU offered to reimburse me $50 and only because I was in business class. Period end of story. They brought the cash in RMB currency along with my lost bag. I'd say 0% chance of getting money after the fact had I not arranged this with their office before the bag was delivered.
DL: I could track my lost bag the entire time with DL and the App, even though MU as the last operating carrier for ME (not my bag) was responsible for delivery to my hotel. I submitted a claim to DL for the remaining $150 and DL promptly issued a cheque to me (not sure if this was because the flight was on a 006 ticket and it was 100% DL's fault for losing my bag). But, they took care of me very efficiently after the fact.

Airport Experience
MU: PVG is a disaster. Total Disaster. Check in was a zoo (even in Sky Priority area). Immigration a zoo. Security a Zoo. And the transfer area was even worse. I missed my connection because MU's inbound aircraft was late, and despite 45 minutes to connect in PVG they wouldn't let me past the transfer desk. No English spoken. None. Was put on the next day's flight and offered a dumpy hotel, which they upgraded to a slightly less dumpy hotel when I complained via a fellow traveler I met in the same situation who was fluent in Mandarin. Ended up moving to my own hotel because their 2nd hotel was just terrible and haven't even wasted the calories trying to get reimbursed. To their credit, my bag was delivered to the bag claim area and I re-checked it the next day. Leave yourself a minimum of 4 hours to check in and go through immigration/security. You need at least 2 hours to transfer at PVG on MU to MU practically speaking.

MU lounges were odd, small, and filthy at PVG. Small lounge downstairs at each gate next to the jet bridge that you could access before boarding. Only you needed to walk back upstairs (no upward escalator) to scan your boarding pass to board, and then walk back downstairs to the jet bridge. Very odd and not sure how they prevent people from sneaking onto the wrong flight.

DL: Didn't try DL at PVG but I'll say part of the issue is PVG, not necessarily MU or DL. SkyClubs are >>>> compared to MU lounges at PVG.

Summary: I would consider flying MU again for a direct flight only, but never a connection, and only if I absolutely had no other reasonable choice. I'd book via DL even though you can maximize MQMs and MQDs by booking with MU directly in many cases. That way DL has skin in the game and may help you if you need it.

Overall DL is a better well rounded airline, though their business class hard product is getting a little dated. Service is infinitely better on DL. If given the choice, DL would be my go to carrier compared to MU any day.

MU has got to improve their ground service if they want to be taken seriously. And English needs to be a mandatory common language given their membership in SkyTeam.