Git Workflow for Multiple Teams

TL;DR: your git workflow isn't really the problem. The problem is that you need more, smaller iterations on the features you put in your topical branches. This will reduce the pain of keeping these topical branches up-to-date and integrating them into the upstream.

You definitely want to keep unmerged branches up-to-date with the changes in their upstream, and rebasing is generally the correct way to do this.

Your comment that "sometimes a feature is bigger than what a branch should be" leads me to believe that you have long-running topical branches that you find difficult to integrate with your integration branch. This, in my experience, is the actual root of your pain.

Imagine if your topical branches lasted a few hours and then were merged back in to the integration branch. These ephemeral branches are likely to be trivial to keep up-to-date and trivial to merge back into your integration branch. On the other hand, imagine a long-running topical branch that spans multiple releases of the software without integration. It would probably be quite difficult to integrate. This should lead you to conclude that short-running topical branches that are frequently rebased against master are easier to work with.

The question, then, becomes "why would features be bigger than what a branch should be?" This is probably because you're trying to do too much at once. The best way to keep topical branches short-lived and to make integration painless is to work in an iterative fashion where the minimum marketable feature is ruthlessly hewn down to its bare essentials and further work on that feature is added on in separate increments.

I like the git flow branching model. The master branch is left alone most of the time, it only contains releases. The develop branch should be stable at all times, and the feature branches can be broken.

You can also combine this with continuous integration by merging develop into your feature branch and your feature branch into develop. Of course you should only merge something into develop when you're confident that things are working and do not break.