I have only attended a few potions classes, but even in those few classes I have realized tat potions is not what it looks like. Potions is considered one of the hardest classes to master at Hogwarts.

Why is potions class so hard?

Well for starters brewing potions requires precision and complexity. You have to measure the ingredients perfectly, you have to put the ingredients in at the right time. You have to stir the potion at the right time, in the right direction. You have to count in your head, because for some potions it has to be on the fire for 30 seconds.

You also have to collect and prepare challenging ingredients. For complex potion like the Animagus potion or Veritaserum, they both have a certain environment to work. They both have to have a full moon at some point. For the Animagus potion you have to have a mandrake leaf in your mouth for a month, and if you swallow it or take it out at any point you have to restart. It's not just prepearing the ingredients, some ingredients in potions are super rare and hard to find. Or they cost a lot of money in apothecary shops.

There are also high stakes in potions. If you mess up a potion even just a little bit it could have serious consequences. for example if you incorrectly brew the confusing concoction, it could result in your robes catching on fire, the potion not doing anything, or it could cause physical injuries.

Also, you have to maintain environmental control. You have to maintain specific heat, preventing reactions from other cauldron materials (metal vs. glass), and precise timing is crucial.

Finally, you don't just have to follow a recipe, you have to understand the ingredients, and how they each affect the potion. It would also help you brew the potion better, and maybe creating your own potion. You would also want to understand magical recaxtion, like if you add this to that what would happen.

So, now you know some of the reasons why people believe that potions is the hardest subject. Did't know well now you do.