Here is my opinion to add to the above:
To me there is no surprise as to the .425 WR cartridge. That proprietary cartridges were the norm in that era, Westley Richards would naturally have undertaken development of a rimless cartridge to compete with the slightly earlier .404 Jeffery. Their client base would have demanded it.
As to the rebated rim? There was a certain fascination with fast stripper clip reloads, as well as saving having to do bolt face alteration.

I have never read or heard of a failure to feed or extract in an "original" Westley Richards quality sporting rifle, whether magazine or double rifle. Likely the same cannot be said for lesser quality rifles destined for game departments, or eventually chambered by other makers.

A properly set up rifle will be as reliable in this cartridge as any other "if" the maker takes the time to get it right.
In the old days (and to this day) Westley Richards took the time to get it right.
- Mike