I don't know why but very often when I drink relatively low quality homebrews, I taste astringency and some phenolic off-flavors that I can't describe very accurately.
I know what you mean. I do think it borders on astringency. It's like a combination too-high ph in the mash, too much high-alpha acid hops early in the boil without the body to back it up and bad sanitation at bottling. I've noticed it repeatedly in homebrews that have been given to me over the years (a couple times in my own to a lesser extent, so I believe). Very strange.
I remember first experiencing this phenomenon a number of years ago when a variety pack was given to me by an "award-winning" homebrewer. I couldn't drink any of them. It was then that I decided that I would never enter a competition.