There's also at least a potential aesthetic problem where the new building behind the facade is taller than the facade, especially if it extends above by a significant amount. That's very common indeed.

I'm happy with that look. If you can't hide the change perfectly, I'd rather the building be honest and blatant about being a hybrid.

But I can see how tastes differ.

It varies in how well it’s done - much as with windows, some are very sympathetic and others not. It’s also particularly noticeable for people who occupy higher floors in other nearby buildings and I’m not sure this is often considered in the planning process.