Relatively low risk, these are open-source projects with fairly significant userbases of technical users who can and do call out troubling code if it's present.

While you are doing so at your own risk, sideloading isn't inherently risky. It's far safer than jailbreaking, for sure. The risk is not really with the store, but with the apps you download from it. I recommend only downloading more popular and trusted apps with public code and some community oversight.

Almost none. Just be careful who you give your data to.