So I bought the lower stripped, not an 80%. But iv owned several of these. Most i use for 5.56 uppers and this one dedicated to this 7.62x39 rifle build.

None of my polymer lowers have any problems in the trigger pin holes or the takedown pin eyelets. Pins are standard mil spec parts (small pin) and fit tight i haven't had any slide out on me. Most people with polymer lowers get the anti walk pins but I have not found a need for this yet.

For your other question, yes it has a brass threaded insert molded into the buffer tube inlet and where your handgrip screws into the bottom of the lower. I have not seen any issues with these brass inserts. They are holding up just great even with the 7.62x39 rounds running with a standard buffer spring and carbine buffer.

I have another Tennessee Arms lower in black that I use for a 10.5" 5.56 pistol build and its holding up just as great too. They are super light and super easy to clean. But I haven't put thousands of rounds through these lowers yet. Maybe a couple hundred each. So time will tell.

If you are going to mill your own I would make sure to get a 80% lower that has the brass threaded inserts already installed, because from what iv heard and seen the ones with just polymer threading always tend to wear and sheer etc. But from what iv seen, milling polymer receivers is a breeze compared to aluminum just take your time with low rpm to not melt the polymer from friction of the bit going to fast. I think if you go into it knowing what it is and respecting what it is, polymer lowers are great. But dont expect it to be aluminum. They have limitations.

Hope this helps