Quartzite is very hard. That's a big difference than "very strong". Hard is also brittle. And prone to breakage in some locations that get repeated dings. Quartzite also can etch when exposed to acids. Not all will do that, but many will. It depends on the particular stone's particular chemical composition.

I know it's disappointing thinking you spent thousands on a product that isn't wearing as well as you had hoped. Unfortunately, I don't see anything but normal natural stone behavior.

Hmm. Actual quartzite will not etch. If your stone is truly etching, then my first guess is that you got a "quartzite" that is actually marble or dolomitic marble. Many stoneyards don't even know the difference, so I don't know if that counts as cheating or as them just being clueless and passing their cluelessness on to you.

As for the rock coming apart, that could happen with either quartzite or marble. As Green Designs mentioned, if a rock is dis-aggregating, that is a result of the rock not being "stuck together" very well. This can happen almost regardless of what the rock is made of. Some quartzites are not particularly well stuck together, some are perfect, and others are so well-cemented that they chip rather than fall apart. So that is a separate issue, independent of rock type.

On the etching though, I'd do some further investigation. Do you have any leftover pieces you can put lemon juice on?

Also, can you post an overall view of your countertop? It sort of looks like quartz sandstone to me. That would be the un-metamorphosed version of quartzite, and yes, that would have a tendency to disaggregate. However we have a quartz sandstone in our kitchen, so I am in favor of that rock, on principle.

Hope this helps!
Karin