Token efficiency is good, but in practice, model familiarity is more important.

Look at what happened with TOON. They came up with a format meant to replace JSON for agents, but despite being way more succinct, its novelty meant that LLMs spent just as many tokens correcting mistakes while using it.

TBF, that was several months ago, an eternity in LLM time. Someone should rerun those language studies, it's possible the SOTA has changed.