I went through 3 coils in 2 years on my ts250 (completely standard 6v points, mechanical regulator etc).
Started to cut out when warm, then wouldn’t start, got worse and worse. I changed everything and a new coil from mz-b sorted it. A few months later same problem and it caused me a lot of bother as I didn’t suspect the coil due to it being almost new. It threw me even more as I looked up the resistance measurements for it and they were all within specifications. It would even give a spark when the the plug was out and resting on the cylinder head but not spark at all when it was in the cylinder under compression and wet with fuel! Changed it as a last resort and no further problems so definitely the coil. The only thing that gave it away was a very slight amount of oil leaking from where the ht lead is fitted into it. I think I also cooked one by leaving the ignition on for a short while without starting it. So the new coil is this case lasted less time than the spark plug. My current coil is a used one off a ts150 project I am building and has lasted 8 months. I now carry a spare new coil and it is my first part I will replace if I have a problem. Most of the “magic” that is the spark is done by the coil, an early warning that your ignition system is starting to have problems is that the engine starts to miss fire when you put all the lights on, including brake light, but this can be your battery on the way out, if you know battery is good then it’s the coil.