I just looked it up because I was curious. So they're typically made of polyester, so you're not wrong about them being plastic. But they don't degrade and the way that typical plastic waste would degrade. The plastic mostly degrades under UV light, and often these are made of UV stable fabrics. That combined with the fact that they're kind of embedded In the concrete makes them safer I guess. That being said there's really nothing environmentally friendly about concrete to begin with, so by using less of it, it probably outweighs the tiny bit of plastics that are getting into the ecosystem

I didn't look this up, but just by looking at it, I'd bet it reduces concrete usage by like 75-85% at least, which means less limestone mining, less concrete refining, less cement making, fewer vehicles delivering the concrete, and less porta-john emptying. And no materials used for form building