If she is merely a share holder, as I am, no matter how large and has no involvement in the company I see no difference. I benefit (I hope) financially from the decisions British Gas makes, and because we treat income coming into family members as family money so does my DH.
The difference lies, not in whether the PM and wife benefit from the decisions the company makes, but whether the PM could make decisions that benfit the company, but we have a system of checks and balances in this country, so that the PM does not make unileteral decisions, Equally Infosys, a very large international company will not want to get involve in scandals if it can avoid it - and when the wife of a PM is a lareg shareholder, you know that you are going to be under intense public scrutiny.
I am not saying the system is perfect, and we saw that during COVID when contracts were given out, without proper checks, but mainly to personal cronie not big companies, and, of course birds are coing home to roost there.