Start loading them with 'use bignum', 'use bigint', etc. (which I regard as the insane way) and they can get a bit tricky
The insanity of bignum
has its tendrils wrapped around Math::BigInt in ways that are not easy to fix without breaking backwards compatibility.
I regard the major drawback to the standard multi-precision math modules as being their speed.
I almost included that in my list of problems, but I think the slowness is due in large part to the overcomplexity of supporting bignum