I had a similar experience to this. I think the difference is that FB was really focused on making sure HHVM ran for their internal use cases specifically, and

I had a similar experience to this. I think the difference is that FB was really focused on making sure HHVM ran for their internal use cases specifically, and broad compatibility was a secondary concern in the early days (and perhaps still is? I haven't looked at HHVM in a while). To be fair, they were quite transparent about this, and published quite a bit of documentation around what worked and what didn't work when running PHP on HHVM so that was good.