In a map like this one, a guy with a railgun may kill a guy with a bfg at long distance, I guess. Anyway, it's a good suggestion to do not make it appear after just 5 seconds like standard weapons.
Everyone agree (even some ID Software guys if I remember well) that the Q3 bfg is totally off-balance with the rest of the game. Even a railgun cannot compete (one or two bfg shot is enough to kill anyone, and is almost instant shot because the projectiles are way too fast), so no there's no competition against this weapon. It has all the benefits without any drawbacks. The only thing to make it the invincible weapon would be to make it a real instant weapon, but it's already kind of instant as I pointed out above.
Anyway, it's up to the mapper to choose. But a good map cannot put aside gameplay, even if it's not made towards a competition use.
@Neon_Knight: please do not be downed by these comments, your map is great and I like it. You're doing a great job mapping for OA, and your work is very much appreciated.