As the title says, MS has completely ruined the Whiteboard app and made it totally unusable. It takes a lot of time to load and when you ink, parts of it keep disappearing. I guess it’s now basically a webapp.
Luckily I stumbled on a good third party alternative, Flowboard in the Microsoft store. Still in development, but it’s what Whiteboard was before the downgrade. U should really try it out.
https://apps.microsoft.com/store/detail/flowboard-firecubes-edition/9PB20HCH5XN2?hl=en-ca&gl=ca
Does it run on multiple platforms?
Yep, it’s barely usable.
I get that having so many features bogs it down, but there’s a line where it becomes unusable.
Flowboard seems decent if all you want is writing/drawing, and it has some nice features for that. But it is missing so much compared to Microsoft Whiteboard that that they are really completely different products.
And no, it’s not available on other platforms.
No, not at this point.
Whiteboard on Android is an even greater mess.
I had that same issue in Word with inking (I think it is fixed now but it was quite annoying).
It used to be great. Long story short, they downgraded it to a web app so it can be embedded in Teams. 
Also of note, it only runs in x86 emulation on ARM devices. Because if there’s one thing we know well it’s how well web browsers run in ARM emulation. And it’s not like Microsoft owns two separate frameworks for running webapps that support native ARM or anything 
(Electron and WebView).
I don’t know where to start with the stupidity there.
If you’re going to do that, why make it a web app?!