[WhoisAlanGreenspan?]

First I’ve ever heard of that assessment, do you have links or support for that argument?

[NewHampshireDuo]

There are more ways to screw up with a twin. A single functioning engine permits you to make serious mistakes.

[blackdog]

Critical engine out stall speed gets deadly. You lose an engine in a twin while climbing out heavy at best rate of climb and you are below stall speed. You roll right over and flat spin upside down in about ten seconds. The Baron for example. Also managing approaches and following ATC instructions while managing fuel, engine cooling, course corrections, and passengers at 220KTS is just way too much too fast for safety with just one pilot.

[caltaxed]

Richard Collins did a study years ago comparing single piston and twin piston aircraft with retractable landing gear and said the twin had 4X the fatality rate (see the link below).

Improved training and other corrective action measures have improved the safety of twin engine piston aircraft but insurers are still leery.

https://airfactsjournal.com/2011/12/50-years-ago-in-air-facts-double-trouble/