I'm a member of four ex Yahoo groups that made the jump to Groups.io.
No great issues reported beyond corruption of a few files, a few photos going missing and an inability to deal properly with mail attachments. Most closely connected with the Bridgeport Mill group transition. Being the outgoing moderator for the Yahoo group I'd expected to be roped in for loose end tidying as the new management got to grips with things but in the event the only thing I had to do was correct three of my own contributed files which had become corrupted and re-direct a few membership requests from the old group to new. We did have the luxury of leaving the old group up, albeit moribund, for a couple of months so anything identified as corrupt could be grabbed from the old data.
Free switching to Groups.io has gone. You have to set up a Premium group to switch which costs about £100 a year. Far as I can see all the old Yahoo groups intend to drop back to the lower tier Free groups after the first year. Free groups are very similar to the old Yahoo and seem good enough for what folk like us want to do. Premium has much greater data storage and many bells and whistle that are excellent if you need them. Data limits on the Free version seem to be more than sufficient. I saw discussion in another place concerning moving a very data and photo heavy group. The owner seemed confident that free would do.
Reading the official "Things are changing" E-Mail from Yahoo central its clear they have completely missed that the files / data / photos store is, to folk like us, a primary purpose of such groups. Yahoo say the communications between members is primarily by E-Mail so no need for the data storage. Well DoH, the E-Mail bit is how we communicate but the steady build up of how it works, how to fix, what to get for, this is what it should look like yadda yadda yadda in the files is why we hang around. On an E-Mail or social meadia based system you end up having the the same question asked umpty-six times until thems-what-know get fed up and down pens (keyboards!).
No data reposity beyond photo albums is a weakness of this forum, search only gets you so far especially on popular topics where the number of returns can be overwhelming. Be a nice topic for an AI mavern to develop a forum post distilling bot able to turn many posts on subject into a coherent "how to do it" / "all about it" et al file. At 65 I don't expect to live long enough to see that sorted properly tho'.
Clive