You can view the complete list of supported and unsupported PGQL features by the graph server (PGX) in Supported PGQL Features and Limitations.

However, the following explains certain limitations for a few PGQL features:

• Limitations on Quantifiers
  Although all quantifiers such as *, +, and {1,4} are supported for reachability and shortest path patterns, the only quantifier that is supported for cheapest path patterns is * (zero or more).
• Limitations on WHERE and COST Clauses in Quantified Patterns