"I've found in P89C669 errata sheet some problems with 9 bit UART "

The inadvertant change of RB8 and SBUF values does appear to be a bad bug. And since you seem to be needing 9-bit comm for your m_bus, could be a problem. Might need a more in depth analysis before deciding to drop the chip.

"And here is also unpleasant possibility to erase the boot vector. "

With a good supervisor chip, having brownout detection, this worry is a thing of past. You can search this forum for a document called No-Touch by Erik which addresses this issue nicely.

If you ask me I will settle for the P89c669.

Raghu