I think most of that is the exception. From what I can see, Powerstack is a marketing name for their newer and "better" battery, and is completely compatible and replaceable with their standard 20V line. XR is a similar story. Since their "20V Max" is really 18v with a new coat of marketing (No, really), their lineup was already compatible. They just had to put a different connector on it. Atomic is a line of tools (smaller), and are also primarily the same 20v

As for the 12v, 60v, Flex, and whatever else - unless you need a very wide variety in your tool collection, you'll probably just be using the standard 20v line. Everything is labeled as 20v Max, which works quite well for ensuring compatibility.

As for quality, I'm not an expert on the subject. There are certainly still plenty of fans out there, and it seems to be one of the better brands regardless of any complaints. It's what I standardized on recently as a DIYer. But it may not be quite as professional-grade as it used to be.

Makita is also a good brand, and I wouldn't object to that either.