That's not exactly how I understand it - my understanding is that they are generally very good in terms of COP but the trade-off is that they operate at much hi

That's not exactly how I understand it - my understanding is that they are generally very good in terms of COP but the trade-off is that they operate at much higher pressures than other refrigerants (including the ones we're phasing out), so material properties, servicing, etc. are more challenging. (of course, it's more complicated than that because COP changes depends on input and output temperature, both in absolute and relative terms, and different refrigerants can be better for colder inputs or hotter outputs etc., but I am talking in general).