The app? There are many, many clients for Lemmy. Thunder is able to handle multiple versions and multiple accounts on multiple instances. This is a client API implementation limitation of whatever client you are using. Not Lemmy itself.
As for the federation issues, they are being worked out. Each major version has improved the internal server logic, improving the reliability of the inter-instance communication. But the changes have also come with kinks, that have had to be worked out with each sub-version update, before the team bites off on the next big improvement.
v19 introduced changes to how the federation queue works, and these are currently causing higher resource usage than before. Because of this, some instances seem to be falling behind on syncing federated content.
With the current tiny team, at least a year out. Probably more.
With improved functionality, adoption will likely improve, with improved adoption interest in developing it should improve... Once that feedback loop gets going things can go very fast.
The reddit migration has seeded Lemmy with a bunch of competent client apps, and developers to work on them (I've personally contributed to Thunder). But it didn't do much for the development of Lemmy itself.
Mastodon has achieved critical mass among developer interest, I think, and is teetering on the edge of becoming a real mainstream option. When I first tried it many years ago, it was near unusable for the average normal person. And while it is now much better, it's been years. Lemmy is a lot closer to the start of that same road.