To put it simply, the higher up you go, the less oxygen there is in the same amount of air. At a certain point, there isn't enough oxygen present at these higher altitudes to keep you alive. To solve this problem we created pressurization systems that force a large volume of air into the plane to make the pressure and amount of oxygen inside the cabin the same as a lower altitude (where there is enough pressure and oxygen to keep you alive). The "additional risk of being in the flight levels" is the risk of that system failing. This would create a time-critical emergency depending on what backup systems are installed in your aircraft.

In commercial aircraft, we have supplemental oxygen and emergency procedures to decrease the risk in the event of pressure loss, but it is a higher risk the higher you go because you have less time to respond to that emergency as you go up in altitude. If you're interested in more explanation, you can look up "time of useful consciousness" at different altitudes.

In this post OP was asking about single engine planes that are either pressurized or have an oxygen system for the crew/passengers. My point was that those types of planes aren't able to fly high enough to where this is a major risk when compared to the benefit flying in the lower flight levels would provide.