The biggest gap I've noticed in SyncThing right now is that their iOS client doesn't work well. It's a third party solution you have to pay for, and it can't sync into app folders or the new iOS file paradigm introduced in recent versions of iOS.
Asking as a Nextcloud (file only) user considering making a switch.
To be fair, I still would really like an official, open source iOS client, instead of some closed source thing I need to trust; I'm still a bit uneasy about using it.