Of course you can. The actual question is: do you trust the author(s) of the repositories you're pulling the APKs from? Including that they are keeping the repo secure from malicious influences? If the answer is "no", then you shouldn't add the repo, obviously. Every repository acts as an individual trust anchor. Unlike F-Droid or the play store, where the store itself acts as the trust anchor (or should, at least)

To be clear, I'm using obtainium for quite a few apps, but I'm rather rather careful which I add there and what apps I'm getting elsewhere.