Flying IMC is a serious matter. I have met several pilots who venture SE unstabilsed to get on top or have done a bit of training in the soup. They are a danger to themselves and others. If a SE aircraft IS certified (some are) that is a different matter: it will have stabilisation and system redundancy.
The UK military used to fly single engined, unstabilised helicopters in IMC. I was trained to do this and also trained others to do it. It requires a lot of concentration.
More recently I flew a UK police helicopter, albeit a twin, that had no stabilisation or trim system. Thankfully it's been retired now and these days I fly approved SPIFR twins with good systems and 3/4 axis autopilots.
Illegal IMC? Yes, it certainly still happens. I had a close shave under the London TMA when an unknown contact (squawking mode A only) was notified to me by ATC, who were providing me with a radar service. As it had no Mode C and was manoeuvring I (wrongly) assumed it was flying in VMC at low level. However, a few seconds later it appeared just underneath my aircraft, probably closer than 100 feet below, in solid IMC, at 90 degrees to my path. There would have been no time for avoiding action from either aircraft, it all happened in a flash. I had taken the option to transit at 2400 feet under IFR and a radar service because the cloudbase was not much above 1000 feet. The pilot was obviously not under a radar service because I was speaking to the only provider. If he had, he would have given the game away by declaring his altitude. I regret not filing an Airprox at the time because it was an R-44. I know the colour, if not the registration, and from which airfield it came.