This feature is (in my opinion) very bad when you think a bit longer about it.
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You add a dangerous single point of compromise to your accounts. When your Proton account is compromised, the attacker will have access to all other accounts that you have linked, without resetting the password as they can login with the credentials they just gained.
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I agree with the others here, do NOT have everything in the same basket. I know it is very convenient to have everything in one place but one thing why Google is such a pain is the monopoly and the vendor lock (you can call it ecosystem but lets be real, it is a vendor that locks you into the products). This also comes back to my first point, single point of compromise.
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Yes Proton is build for privacy, even is now switching to be a foundation with solid share holder rules etc. but we do not know where Proton will be in 10 years (it is like with governments, do not give them the tools to control you just because they are nice now, they can change). If Proton would change into a more data collection company/foundation they would have access to many more data points. All webpages/services you linked your proton account with, track pixels etc.
Let's beat Google by being more respectful to users privacy and not by just copying them, let's be better!
I'm still in the world and character building phase. Sketching landscapes, rooms and such, writing little short stories to form the characters (my idea is that they have a "memory", don't know if it works in the end).
As for now I'm completely analog, I bought a nice notebook and do everything in it. But when time comes I think I will go with Obsidian as I already use it for my other notes/projects. I think it will be very powerful with the graph-view regarding relationships etc.
Obsidian is not FOSS though, and there are alternatives. I just hadn't the time to look at them.
Edit: Obsidian can be used for free when you do not use their sync features. I use it that way and just have it in a Git-Repo for accessing it on different devices. Obsidian notes are in Markdown so you also do not have some kind of vendor lock, it's just about the links between the notes.