Fitted correctly, for the right application, there's nowt wrong with a square section spring.

But a badly installed square section spring can cause some horrific damage to the inside of a cylinder. Generally as a result of too much preload, and a badly supported spring.

I had a FWB127 in, that I was asked to service a few months ago, that was chewed up so badly, internally, by the poorly fitted spring, it was scrapped and broken for parts.
As soon as the spring moves anywhere out of true, on compression, the edges will simply dig in and scour the cylinder. This also occurs with round section springs, but as they have no flats / corners on the coils, they tend to do far less damage.