... other than the fact that the proxy can view, record, and log all your traffic?
I don't consider this a small risk already since the ability to view the traffic also includes the ability to steal credentials, session cookies etc. But it gets worse: the proxy can actually modify the traffic.
If you use only HTTPS and don't install some certificate which allows the proxy to intercept this traffic then this is probably less a problem since HTTPS protects against traffic modification (i.e. at worst data transfer will fail). But if you only use a little bit of plain HTTP (which is very likely) then the proxy might modify these - for example injecting ads, modifying downloads in-place so that they are infected, redirecting you to some phishing sites which look like real sites (but are not) where you enter your credentials ...
At the end you should ask yourself what the business model for the operator of the proxy is. To run such a proxy money and time are needed and the better the proxy is (bandwidth, speed...) the more money is needed. Since only few have money to throw away there is usually some downside - if you don't pay you are the product.