As others mentioned above it is much harder to estimate distance and relative velocity to a flashing light that to a constant one. From my perspective the best rear lights shine from a bigger area instead just from a single (LED-)spot. My favorite is the Philips Lumiring, but unfortunately Philips canceled their bike lights production. However, the German brand Bumm uses the same technology in some lights and advertices it with "Line Tec".

There is a huge difference between detecting an object and identifying an object. Flashing, annoying lights make it easy to detect an object, but it is almost impossible to identify it. Recently I almost crashed a dog, which had a flashing collar. I detected the "object" from quite a far distance, but mixed it up with a bike rear light. However, I only identified it as a dog, when I was just starting to overtake and I would have had crashed the dog, if it had decided to cross the street, just when I was passing. (Since I expeced a bike, which is not able to go left/right instantly.)

From my perspective, detecting a bike light with a working rear light, is not a big issue. Identifying it as bike is more complicated and estimating its exact position and speed, is a security relevant issue, which is much more difficult with flashing lights.

I don`t know the technical specifications for StVO-approved rear lights in detail. However, this is not needed for the consumer, since any approved bike light has to have a specific number "K~~XXXXXX" on the product and in Germany any light sold as bike light has to have this certification. So the consumer does not need to take care on that issue when shopping a light - all leagally sold bike lights are StVO-approved.