I haven't used the folder option, but but search seems to have worked well enough for me. I believe the client at least is open source. (That is, when updating on Arch-based Linux, it's going through a JavaScript-based build process. Should all be there somewhere.)
Their selling feature (to me) is the privacy focus, because end to end encryption is the default, and a very rare one among other note apps I've taken a look at.
Sync works well, and is one way I use the app, even when I have other things like org-mode in play on some devices. It is typically quite handy to just paste something on my Linux desktop, and copy-paste on my iPhone to send somewhere iOS-specific.
Folders requires a subscription on Standard Notes as well.
I only use the free version of Standard Notes, but I am a big fan of the product. Their syncing has been the most solid/problem-free I've seen amongst all of the notes apps I've tried over the years.
My bigger gripes are that it doesn't support (markdown) formatting without a subscription, and I can't figure out how to put notes into folders.