Mehdi Ghapanvari explains that columnstore indexes should not be a default:
A SQL Server columnstore index does not improve performance when a query fetches many columns. This is an important factor to consider when choosing between columnar and row-based data storage. In this article, I will set up a demo to show this point.
Yes, it’s obvious if you know how columnstore indexes work. But if you’re new to the topic, it’s a good primer on why we don’t use these things everywhere. But in their wheelhouse, they’re incredibly powerful.