- Is it guaranteed that all compliant compilers have the same experimental features?
No, experimental features are optional.
- Are experimental features prone to big changes that make them unreliable?
Yes, the C++ committee might even decide to abandon a feature or in the process of standardization a defect might come up that would force a feature to change.
Generally, it’s not a good idea to depend on experimental features. Experimental features are exactly what the word says (i.e., to experiment with).