The gears are a real pain, but when set work wonderfully. I own a 50's/60's enfield that's done more than a few km's and the stock gears are fine, but the issue is setting the shafts with the correct spacing to keep the valve lash down and the inability to compensate for this over time. As soon as you start getting lash, the accel/decel causes wear, and you tend to add up the last on the distributor drive. If you look on the cutaway, you can see that the pinion drives the first cam, which drives the second cam, which drives both of the idlers, which drives the distributor gear. Add a degree of backlash to everything and all of a sudden you have 5 degrees of backlash on the distributor drive, which means your timing varies by 2.5 degrees on accel/decel (not to mention that the advance unit goes nuts when you take up the slop in either direction.
Compare this with the inter and other twins that just do the chain drive which solves this as long as the tension is set right on all the chain throws.
But on the other hand, bullets have been around for ages, and system works even when it wears out and has a ton of slop! That's basically the principle of the bullet in a nutshell