Because they're disparate types of PSCs and you'd get unpredictable behavior. Even still, it's not supported, so don't do it if it's for production.
what is the situation when it comes to Inter-Sites replication
Embedded PSCs totally supported on each end.
If we've a VC with external PSC in Site-A same as in Saite-B can we replicate also?
Yes
so Inter-Site replication is supported, plus we may leverage NSX to provide L2OverL3 so it acts as Intra-Site