TypeScript has two kinds of private and they fail differently: when each one bites

The keyword and the hash are not two spellings of the same idea, and choosing by taste causes trouble later.

The keyword is compile-time only. It disappears entirely when compiled. At runtime the property is an ordinary one, visible to anything holding the object, enumerable, and reachable from plain JavaScript or a bracket lookup. It is a rule the compiler enforces on you, not a property of the object.

The hash is a real runtime private field. It is enforced by the engine, invisible to code outside the class, and not reachable at all from outside.

What that means in practice

• Testing. A keyword private can be reached from a test with a cast. A hash field cannot, ever, so you have to test through the public surface. Some people consider that a feature.
• Serialisation. Keyword privates appear in output when you convert an object to JSON, which surprises people and occasionally leaks something. Hash fields do not.
• Structural typing. A class with a keyword private stops being structurally compatible with an identical class, which is sometimes exactly what you want and sometimes the reason a type stops matching for no visible reason.
• Copying and proxies. Object spread drops hash fields, and proxies need care, because the field is tied to the actual instance.
• Library boundaries. If consumers might be plain JavaScript, only the hash actually protects anything.

Reasonable default: hash for genuine invariants and anything that must not be serialised or reached. Keyword for internals where the goal is communicating intent within a codebase you control.