Hi Michał,
I hope some others will join in, who have experience with this.
You have probably read the knowledge base post going SharePointless
https://community.webcon.com/posts/post/going-sharepointless-how-to-transform-your-environment-into-standalone/168
So you are aware of the no longer available SharePoint groups, which you probably already left behind since you are using BPS 2020 on BPS Portal.
The only other difference I'm aware of is the editability of office documents. With SharePoint multiple users could simultaneously edit a document. This is not possible with BPS Portal. But it is possible to edit a document without downloading / uploading it. BPS Portal supports also WebDav, which is used by SharePoint too. See this post for a little discussion about it:
https://community.webcon.com/forum/thread/423?messageid=423
There's one other information, which a little bit related.
The BPS_Config database contains a global parameter CloudComponents. This lists components which are disabled in the cloud.
Also I can only guess what they mean, I would assume, that these actions aren't supported in a standalone installation, too:
ImportExportWebParts
SharepointListContext
GenerateProcessSite
Best regards,
Daniel