Some interesting new features can be found in this section, but if you are looking for all the new features for each specific version starting from version 5.1.1, you can find them in:
Cloud:
5.2: News — Qflow 5.2 documentation
On premise:
5.2: News — Qflow 5.2 documentation
5.1.1 News — Qflow 5.1.1 documentation