The function is actually implemented with quaternions so you might get different results to simply adding together rotators in some cases.
Actual code:
FRotator UKismetMathLibrary::ComposeRotators(FRotator A, FRotator B)
{
FQuat AQuat = FQuat(A);
FQuat BQuat = FQuat(B);
return FRotator(BQuat*AQuat);
}