My only issue has been the lobby lag, which only begins for me around 10-11am EST when Blocks really begin filling up.
I suspect this will improve as time goes on; as the initial influx wave wanes.
What won't likely soon to improve is the wider experience for those having genuine technical issues. The Xbox play-anywhere experience isn't being kind to a lot of people; as this isn't a standard run-of-the-mill PC product release, but something i feel is still in its infancy- it's a combination of cloud-gaming and digital distribution.
But it is Microsoft, if there is one thing I've learned from using Microsoft products all these years, is that as time goes on, things do get better; it's just a question of whether the consumer is patient enough for that to happen.
I can't really speak for SEGA though, my only experience with SEGA is a bygone era of my formidable years.