It's never this simple. You have to decide what subset to use, understand how that subset could interact with the avoided language features, have a strong team

It's never this simple. You have to decide what subset to use, understand how that subset could interact with the avoided language features, have a strong team culture that can agree on the subset and have the discipline to commit, understand how new language features affect that commitment, etc. In addition to needing to read and interact with other libraries that probably use it anyway. The team aspect alone is a deal breaker for me--when you have a technology that attracts people who are intrinsically interested in the technology, good luck trying to get them to avoid certain parts of it. That's a losing battle.