Actually, for greener rolled oolongs like jade TGY and gaoshan, they are a poor choice because the leaf expands so much.
Just based on size, and ignoring factors like clay and firing temp, I think very small pots are best for traditional high fired rolled oolongs, aged yancha with already broken leaves, and aged gong ting cooked puerh. But, as long as the leaf fits, no reason not to.
Personally, I use my 40ml pots for traditional Chou Zhou style preparation of high fired TGY. Since this style requires almost filling the pot to the top with dry leaf, the 40ml makes a great size for one person without breaking the financial bank for high quality tea.