I am also curious about converting the other direction. For, e.g. an clean install started on PostgreSQL transitioned back to Mongo? Not done any diggin' but since you're asking... might as well add mine to the query. 😜
Kinda depends on which php forum. MongoDB uses more ram than mysql.
I can definitely run MyBB lighter than Nodebb on my hosting. I believe that IPB and VBulletin don't run well on shared hosting, so I'd put them about matched with NodeBB in usage. Of course, its been so long since i tried running either of those i could be full of crap.