Sometimes its fine.
Other times it's madness or pulls in a massive polyfill. That's sort of inherent to making a transpiler: they need to match the spec. I usually just need one specific behavior that could be written in a 3 line helper function.
In loose mode they might not follow the letter of the spec, but can cheat for speed or size.
Only thing I can think of (since changing the default to be non-spec would be odd) is just warning on larger code size outputs and saying you could try loose mode in certain plugins/situations. Lastly would be do start creating a tool that does runtime checks to determine how small the compiled output should be