Hi,
In the past i participated sometimes at
Monitoring Portal . They changed to discourse 1-2 years ago. The usability seems fine for me. Since discourse is based on ruby on rails(RoR), the resource requirements are higher than the php based vbulletin. I had never run discourse on my own so far. I just had a quick look when searching for a forum solution, and discarded it, because it would have required a newer stronger machine. (Statement of a former programmer-colleague on ruby on rails: "i do not care about memory - developer time is far more costly.").
I had a glance at nodebb, which was very quick in terms of response times. I liked that very much.
Home | XCP-ng forum is based on NodeBB. NodeBB is based on Node.js.
phpBB has gotten another major rewrite with version 3.3. I used it as an internal communication board(version 3.2). The administration felt very awful back then. then Since it's fully OSS maybe they constantly improve over time?
All three packages are fully open source.
regards