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.
Honestly not entirely sure about iOS. I use a PiHole and several other adblocking solutions, but don't personally use any apple devices. Family members with iOS devices have repeatedly commented that they are confused and delighted that they never see ads at my house, though.