I have a huge database (45gb of research) and it syncs flawlessly for me. Search and sync is almost instantaneous and I’m using iCloud. All my database is stored on my iOS devices, so it took a while to sync first time but now all updates occur as soon as I open the app, and are done after a second or two.
Also, I think the new UI is superb. I tried several alternatives because I was getting tired of the old interface, but ended up returning as I missed the features, especially when writing. Now, it finally feels like a modern MacOS app,