Are Shimano saint pedals suitable for trail / xc use?
In the shortest way of answering this, yes, they will be suitable for trail/xc use.

All flat pedals are relatively the same. They only differ in the amount of pins, contact area, weight and their thickness.

You'll want something on the thin side for trail use, as you'll be pedaling quite a bit more than if you were on a downhill bike. This will help to reduce the amount of pedal strikes.

The weight is probably the biggest difference between all of the flat pedals. It is totally up to you as to what you find acceptable. For everyday trail use, the Saints should be fine and they should last quite a long while.