I would say the problem is that this feature is incredible hard to manage, if you for example are not creating your own UI and port that over from version to version, every Chromium update is breaking the code part you are adding, that gets tiresome and over complicated with time.
That is the main problem if you just change things in Chromium base without doing a complete own implementation - you are depending on good will of Google and when they make things too difficult or change things or remove code as they quite often to to make the browser more minimalistic and simplistic, you have to question the time and effort you are spending into a feature.
That is the sad way how it is
But even if you create a totally new UI (there is only actually one Chromium based browser that does that and offers never before seen features on Chromium platforms) Google makes it harder and harder to maintain that. The only real way to stay safe is to fork off Chromium codebase and develope on your own and backport necessary features and security patches in a manual way.
If you don't you lose customization, features, NPAPI support and so on... the list is already long and will get even longer in the future
