I think GP means that you cannot try out Mint in existing toolchain/codebase without rewriting it from scratch with Mint, which is a deal-breaker. The main selling point of VueJS and Elm (if I remember correctly) is that you can try it out in your existing code (even though with non-trivial setup).

Another problem with a framework especially for UI, is that you need a network effect, that is really a good chunk of people who had tried it in production and could vouch for it, so that other folks can atleast give a thought about giving it a try. This is important given the rate of UI framework churn and JS fatigue that people have gone through in recent times.

Personally, as an UI engineer who has familiarity with almost all of the existing libraries/frameworks/toolchains this looks refreshing and would not mind trying it out.