Update
We've been playing Mystara for a few months now, using Classic Fantasy (the yellow one, not the Mythras one) and we're having a lot of fun..
Everything goes smoothly.
The only thing is the fights, because with the possibility of parrying/dodging sometimes killing a monster with 10 hit points becomes a bit long (but not too long).
But it's beautiful like this and we like it!
We use hit point tracking along with the "total hit points" option, the PCs have a lot of HP and seem invincible, but the possibility of being seriously injured or incapacitated adds tension to the game (without too much mortality that would make them too scared)
BRP is always better, no matter what you play with it... love!