The main downside is that there is a lot less customization of filters short of using a different DNS. There is also the potential for logging DNS (present with normal DNS servers as well). LibreOps claims they don’t log requests, and personally I don’t think they have much reason to lie, but there is still that element of trust. Many of the more well known DNS servers don’t offer ad blocking DNS, so you’ll most likely be switching to a different provider.