Sure, a BigInt has arbitrary precision, and there's a limit to the BigFloat precision - though that limit can be altered to suit.
Does bignum really want to be involved in programs where bigfloat precision fails to preserve bigint precision ?
I've had a gutful of that type of consideration with perls whose nv fails to preserve iv. It's not much fun, and I see nothing to be gained by electing to jump through that hoop.