Unfortunately I have the Zcombo units in three bedrooms and garage, and two Nest Protects (kitchen and hallway). So a mixed bag, and only the Nest units are interconnected. I was on just starting to replacing all my Zcombos w/Nest Protects when Google killed things and so stopped part-way along as I didn't want to end up w/the Nest units and no Smarthome integration, and didn't want to have to rely on listeners.
So I'm currently waiting to see if we actually get some sort of approved Nest integration option from Google, in which case I'll replace all my Zcombo's w/Nest Protects and be done and have an interconnected and HE integrated solution. Key issue is that my family strongly prefers the Nest units to the Zcombos, due to the friendlier voice messaging on the Nest Protects, so an option that uses them what they really want.
If Nest integration doesn't happen, I'm thinking I'll either find a Zwave interconnected system and replace everything w/that, or if that isn't a good path, get a dumb interconnected system (as you noted) and use one or two of the Ecolinks (for backup/increased saftey) to get me HE integration. Either way I'll have a fight w/my family about losing their beloved Nest Protects, but I'm tired of having a mixed system that doesn't communicate, and clearly it's also not as safe as I'd like it to be.