he/him 
Hot take: JavaScript transpiled features should be as spec compliant as possible, even if it ends up being 10x slower. Ship the slow output to older browsers. Using spec incompliant transforms limits your ability to turn them off safely later.
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Syntactic features, yes. Polyfills, maybe. It seems that in many cases you can ship a way simpler polyfill that only implements the subset of a spec that you use, without paying bytes for the edge cases you don't. Harder for a tool to do automatically, though.
You shouldn't ever be inaccurately polyfilling code. If you only rely on a subset then use a different abstraction. You cannot guarantee that people will only rely on the subset of the semantics you have. @ljharb has been pretty vocal about that with es-shims.
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I also have anecdotal evidence at FB. We have custom polyfills that are a subset of semantics and we're struggling to turn them off. Stuff that might not have even been intentional. For example, people consuming something that should be an iterator as an array.
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If you call it a “polyfill” or a “shim” (or any made up nonsense word that implies these) then it must be 100% compliant. Else it’s a “sham”, or, it’s a function full of lies. If you want to make your own decisions about what functionality you need, JS has “functions” for that.
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