Having just purchased a finnair ticket on AA metal, I was able to immediately see my flights in the app and book MCE seats as usual. The one downside I’ve seen so far is that when AA cancelled my return flight, the AA agents couldn’t touch the ticket to rebook me, and I had to go through Finnair. The Finnair agents were very rigid in the options they’d allow for rebooking- I’m pretty sure I would have been given more leeway by AA (based on past experience with schedule changes on AA tickets). There may be other downsides that I haven’t experienced yet- this is my first time booking through a partner.