The shields you find at the very beginning of the game, have the exact same AC as those at the end of Act 1 (+2). Wouldn't surprise me if the 'best' shield only has +4 AC.

Just to further illustrate your point to OP, I'm deep into Act 3, nearing the end of the game, and I've found a single +3 AC shield in the entire game. Even the super late game bosses drop a shield with the standard +2 AC and some unique and powerful effect, rather than a "better shield".

Yes, there are unique weapons you can miss out on. None of them are so unique or powerful that your experience will suffer greatly because you missed it. In one case, a side quest allows you to craft 2 of like 7 potential pieces of equipment and doesn't give you enough material to craft any more. That's the nature of the game: you have to make decisions and work with what you find. On the plus, this makes sharing the experience with friends and playing the game multiple times a substantially better experience.