I think equipment reliability should only be a part of the equation.
For example, if you have an engine failure in a twin while IMC and have never trained for an engine failure, you could easily screw that up.
Similarly whould autos be at least entered while on the dials? How many here have tried to enter an auto on instruments?
Also, even though many with GPS's are ignoring this, but aren't we recommended to alter our course slightly to try and ensure a safe force landing area whenever possible? This couldn't happen if flying IFR.
I think SE IFR is safe enough to do, but if I could, I would always choose a twin to fly IFR. The only reason I could see for allowing SE IFR is to keep it cheap for the operator. That's reasonable, but aren't many of these rules in place to keep the customers safe from the operators?