I havent heard of saddle rails breaking for many, may years. I wouldn't use a carbon road saddle marketed as ultra light on MTB, but i would use a "regular" Ali carbon saddle for MTB.
If you break it fast enough, you can get aliexpress customer service to step in, so you have an incentive to stress test it heavily. If it holds on, it will probably last forever.
Edit: saw a picture of you. You're carrying a few backpacks worth of excess weight, forget what i said. idk how saddles behave at these weights.