There is nothing wrong with single engine IFR, just the same as there is nothing wrong with single engine over water or at night. It is a personal risk assessment choice. Some are prepared to do it others are not, the aircraft does not know it is in cloud or night etc.
Saying quietly to yourself "That's a risk I'm willing to take" is all wrong. Aviation is not about taking risks; it's about identifying them and eliminating them by leaving yourself an out, changing your course, delaying your flight, taking extra fuel, preflighting better, exercising conservative judgement, etc.
Single engine night work and over water is equally foolish. However, single engine IMC is another matter entirely. Particularly in light piston engine airplanes. Setting aside the obvious issues of engine failure, most light singles have one electrical source, limited instrumentation, a single vacum source, no backup instruments, etc. Many of the vacum powered instruments in use tend to precess, some badly, and are often cheap in nature. Fine for training basic procedures on a good day with a hood; poor choices in the weather. Add to that a lack of radar and weather capability, lack of deice, and yes, one single engine. An exposed alternator or generator, exposed to the elements. Unprotected flying surfaces. Very limited performance.
If you've never hit embedded convective weather, you may be in for a surprise. If you've never suddenly encountered a rapid, severe ice buildup, you may be in for a suprise. If you've never suddenly found yourself on a partial panel, experienced an electrical failure on instruments, had a gyro slowly die on you (insidious, and can lead you into the ground because it looks quite normal), fixated, been distracted, been rolled in wake, or any of the other things that can happen, especially more so in a light single airplane...then you may certainly be in for a surprise. In fact, as a willing self-proclaimed risk-taker, you may be found with that surprised look still on your face.
Don't be that guy.