When the bolts securing the cam gear to the camshaft come loose, the camshaft stops turning and things get nasty and expensive.
The standard bolts are more than adequate. There is a problem with them coming loose, but not breaking. The main reason they come loose is that there is nothing locking them (standard practice these days – modern technology doesn’t always go forward!) together with unusual harmonics in the VVC valve train (ever noticed the damped cam gear on the exhaust cam?).
Simple solution – torque to the correct figure (65N-m for the 10mm bolts) and use a good grade of loctite to lock them. End of problem.
Some engine builders, having heard there is a problem with the bolts slackening off, over tighten them. This can cause them to break, but when they are torqued to the correct figure I have never seen a problem. Problem comes when people over tighten them at some stage to “stop them coming loose”, then they get weakened. After that, breaking is only a matter of time.