You need to look at the flaws of individual characters, not the species itself. Although, even the species itself isn't perfect. Your description poses them as perfect shapeshifters who can take any form, human or animal, but that's really far from the truth.
Shai, Swain, and potentially Nidalee*, are examples of Vastayan Shapeshifters; their blood is so muddied between human and Vastayan that the magic in them allows them to shift to blend almost seamlessly with one or the other. Not all Vastaya can shapeshift, just those who have a lot of both human and Vastaya in their blood.
There are also those with /some/ human traits and some Vastaya traits that can't shapeshift: look at Xayah and Rakan - and ostensibly Ahri. They're not "pureblooded" Vastaya, and as such are much more humanoid in form. They can't shapeshift**, but they know a lot about the natures of magic.
And then there are the "pureblooded" Vastaya - examples being Rammus or Volibear, essentially sapient animals. Though, Rammus and Volibear aren't as chimeric as the Vastaya shown in the initial reveal of them (like the monkey things or the otterfolk), but they're still purebloods.
*Not sure if Nidalee is a Vastaya or a Shaman like Frey (from Vayne's lore) - details unclear, and I probably missed something. If anyone could clarify for me whether or not Nidalee is a Vastayan, that would be great.
**Ahri's most recent updated lore has her initially as a fox, although through sapping souls and life essence from people, she gains human form - it could be said either way that Ahri is a pureblooded Vastayan shapeshifter or that she's a half-kind like Xayah and Rakan, so, take what you will of it.
As for individual Vastaya flaws, let's look at some:
Rengar - single-mindedly focused on the hunt and being the greatest hunter alive, and, at one point, grew weak and bored from not having thrilling hunts. It took Kha'Zix to re-sharpen his sense of pride and his desire to hunt.
Ahri - Took souls indiscriminately, though she developed a conscience and started to put her essence-theft towards more productive uses, like fighting off Noxian incursions.
Xayah - (somewhat)Misguidedly antagonistic towards the entirety of the human race, wages a personal war to unleash the magic of Runeterra, which apparently taxes her to no end.
Rakan - Given almost completely to fey moods (bursts of wild, capricious action), kind of dumb, though still very knowledgeable, and, he's driven mostly by passion and love - sometimes a good thing, but also sometimes a bad thing.
That's just a few examples; there's definitely more to be found.
To summarize: You have to look at the race holistically, not just gloss over details and say "they're too perfect" - and even further, you have to look at members of the race to give context to the whole of them.
