Horns on a uPVC window frame.

When replacing sash window frames with a uPVC frame which isn't a sash, but appears of similar dimensions to a sash, it's possible to have horns added to the uPVC frame to resemble the horns of a traditional sash frame.

Are there any problems with these uPVC horns, which someone considering having them installed should be aware of? For example, perhaps are they easily broken?

Thanks