I went to La Clusaz a couple of years ago and did a lot of biking. At the time there were 3-4 DH runs but the red ones are of a similar level to places like coedy brenin so you should be fine on those but not ideal if your on your own.
There are lots of XC route, most good but a few are just average. In places its pretty technical. The biggest loop utilizes the lifts (lift passes are cheap) and is really good with a few brutal climbs.
I think you'll be ok if you take it steady, maybe get some knee pads and a full face if you think you'll be tempted by the DH runs (I would). Compared to places like Morzine there are not many people about on the biking trails so if you have an accident you'll probably be on your own.