Sometimes it is only in the last 10-30% of the changes that any significant benefit is realized, yet the preceding work is a necessary foundation. That makes it a very difficult sell to the rest of the organization (whether OSS contributors or a corporation). Until the last bit it looks like you're making a neverending stream of pointless changes.

Large-scale changes also suffer from "too many cooks in the kitchen" and bikeshedding that can make getting the changes accepted a real slog. It is difficult to get people to buy into your vision purely with words, either because they don't see the value in it or they reflexively disagree that the benefits are achievable (sometimes motivated by their own burnout or cynical attitude).

The solution is to work on it in semi-secret until you can get things to a point where the value is demonstrated. That bypasses endless bikeshedding and pointless feedback rounds because everyone can see the value of the end state (if you've chosen something worthwhile to work on).