I've repaired tons of these. Helicoils are totally up to the job. However as always, the quality of the repair will be down to the operator of the tools.
Timerserts I suppose are better. But once you see the price, you'll probably do what most people do and use a helicoil.
A properly installed helicoil will easily take double the recommended torque. If you're looking after your own bike or using a workshop who knows you don't need a breaker bar to tighten a sump bolt, then a helicoil will be fine.
And DON'T use a stainless steel sump bolt. These are the reason so many threads are stripped. They're too hard and the threads are often too sharp. Especially the Chinese ones. (And which aren't chinese ?) BMW use a softer steel sump bolt for this reason.
However. Whichever you use, your MUST MUST MUST get the thread straight. And if done by hand, then you're always going to be a little bit off compared to a machined thread. If it's slightly pissed, it will leak. Use a thicker and bigger diameter aluminium sump washer when the repair is done. It will reduce chances of leakage. Don't use a copper washer. They're harder and need more torque to seal than aluminum.
The hardest part about the job if you have a non-removable sump like most BMW's is that you can't fit the drill under the bike. So you need a short drill. Or put the bike on it's side. This is where an old mattress comes in handy
