I looked at my account, and I am charged $10 but it seems they automatically moved me to a "Plus" plan that has more storage. So no complaints from me really. Either that or the $4 plan is new. [1]
The $4 only comes with 1GB of storage. I would recommend the $10 for 50GB if you use images in your notes.
And the notes are all just markdown files. If the obsidian software were to disappear you have all your notes. It's fine someone wanted to spend a load of time writing their own software but none of the reasons presented in this piece make sense.
As long as I'm paying for it, I'm the client and not the product.
I tried some of the third party stuff, iCloud, Dropbox, etc and with all of them I either lost data because of notes not being in sync or had to manually fix stuff. $4/month and zero issues was well worth it for a tool I literally use every day.
Why would anyone ever want to use more than one vault? I just use different folders. The only reason I can think of would be if you are using Obsidian for work where you aren't allowed to use unapproved services.
I use one vault per video game. I don’t need to clutter up a vault just so I can link from a page on Art combos in Xenoblade games to a page detailing all the random things I need to pick up around Hyrule to upgrade this or that piece of armor.
There are also several Obsidian community plugins for sync, I use Remotely Save via WebDAV.
I myself currently use Google Drive with DriveSync on Android to sync my notes, which works great. Other cloud providers also work well.
I wrote a comparison of different tools to sync here: https://bryanhogan.com/blog/how-to-sync-obsidian
I also had this nice surprise literally today, all my synced folders stopped working and had to be re-added: https://github.com/Catfriend1/syncthing-android/issues/1430
But overall it has worked well, I am using it between 4 devices.
I previously rolled my own notes system and I find Obsidian plus Syncthing is better. Plugins are a big deal.
I'm using Syncthing [0] to sync my vault between devices. On my main PC, Syncthing runs constantly in the background. Say, if I made a change, and want to send those changes to my phone, I open the application on my phone and let it fetch the changes. It's not perfectly smooth, like Obsidian's own integration, but I prefer this instead of setting a Git repository. Also, the files don't stay in a remote server.
[0]: https://syncthing.net