The "love potion" was actually just a strong aphrodesiac and made from ingrediants of tooru's world, not really magic.
And the only time they polymorphed a human was transforming Kobayashi into a guy. Transforming a human into a human is a lot easier than transforming a dragon into a human. Iirc early on tooru explains that its difficult because a dragon's essence is so big and powerful compared to a humans that its difficult to compress it down into a humans tiny weak form (which is why human form is uncomfortable to dragons, like being squeezed into too small of clothes).
The only dragon to have any notable difficulty with it is Iruru, and its been said before that different dragons have different skillsets and she's notably bad at transforming herself.