Uniswap are huge as a platform at this time, with total value locked around 3.4b$ and daily volume 1.62b$. The only way to shutdown it and loss the value of $UNI is if theres security leaked which enable someone hacked the platform.
It is not because they have a huge daily volume that they can't turn into a scam. Not saying it will happen, but there is always a chance....
Its only a small chance to be scam, its been run for long time ago. Its a decentralized project, not a centralized too, the distribution of $UNI also seems good.