WR, can you back that up? Because if there is one twin that has a very marginal performance when flying on a single engine, it would be the cri cri. And I think the cri cris engine were not aero engines by origin, but came from chain saws. So they would not have the reliability of your average aero engine too.
Granted, you can land it in spaces that helicopters would probably stay away from, and the cri cri is not your average IFR steed, but still...
I even read an article that when the builder/owner of the jet engined cri cri wanted to start flying his creation, he had to get a multi jet engine license first. As the cri cri is a solo machine, he could not get his training on the cri cri. So he had to pay for a 500 USD an hour fighter trainer (don't remember the type, but looked like an F15 or so) to get the appropriate license.