.....the reason I love it is because I associate with it. I had one back in 1982 and it started my love affair with them that still continues today. I say buy whatever takes your fancy as long as you can afford it. Cheers
These are very wise words.
Especially on the internet, we get far too caught up with what is the "best", and then we often miss the real point of VMX: Having fun on bikes you like.
To answer your question:
The RFVC motors love to chew cams/can bearings in the heads, and to crack the heads. The older motors are more durable, but also weigh more and don't go as well. They also have too-short swingarms (to compensate for the lack of grunt from the big, heavy donk) which makes them handle funny.
Yes, they can work, but only in a sow's ear kind of way.
That said, there was some bloke at the National Titles on an XR500 a few years ago, running in the top three and making a lot of good riders look silly.