As I have posted elsewhere around 80% of all engine failures are directly caused by the
actions or inactions of the pilot. Running out of/mismanaging fuel and carb ice are the leading cause of engine failures, both totally preventable by the pilot.
I have done some single engine for real IFR and I want to see 500 feet AGL and 1 mile vis in flat country. For over mountain IMC the cloud base needs to be 500 feet higher than the highest ground with 2 mile vis. I feel the probability is high enough that I could make a survivable landing even breaking out at only 500 feet, that the risk is acceptable.
Frankly of more concern with single engine IFR is you generally only have one vacuum pump and one alternator and generally no de-icing and weather avoidance tools. Those limitations IMO represent more of problem than the one engine.
This reminds me of a conversation at the flying club one rainy nasty day. One guy had just made the statement that he would never fly IFR in a single, when a very experienced retired ATPL walked in. The fellow than said
"Bob how many hours of single engine IMC time do you have?"
After pondering the question for a minute he replied
" I guess about 10 hours"
Another fellow then piped up
" No, he means single engine IFR in an airplane that only had one engine !
"
The reply from Bob was instant
" Oh I have never flown IFR in a single"
2)