Congratulations @Wimpy. From your own words, this is clearly something you wished and as such it has to be a good thing. So, it's always fantastic to see someone see a portion of their wishes coming true.
I'm critic of many canonical engineering decisions, concerning the direction of their flagship product, so forgive me if I am am not so excited about the ability you perceive you will have of bringing into MATE any added value from your experience on Unity. I think that as the development of Unity continues on the direction it seems to be taking, MATE and Unity will become more and more separated. And I believe that at some point you will have trouble finding enough anchoring points to bridge the vastly different workflow between both DEs that can somehow benefit MATE. To me, it seems inevitable that at some point any attempts in that direction will only compromise the ideals behind MATE.
I do believe however that your professional experience on Canonical will make you an even better programmer and project manager, which is what will impact us most if you stay true to MATE ideals, which I trust you will always stay. After all Arch and Ubuntu support are your brainchilds and as a core member of the MATE team no one can deny your commitment.
But if I am a heavy critic of Canonical engineering decisions (and also of a subset of their business decisions), I am not of the company as a representative of good will and as an inspiring and healthy place to work at. The company principles are embodied in Mike Shuttleworth own personality I think. Did you know Mike's almost single-handedly is saving S. Tome and Principe economy? He has invested on this tiny insular nation a large deal of money on a risky adventure to revitalize this Atlantic paradise, through the tourism sector. However, he's doing that not just with an eye on personal profit, but by actually supporting the creation of sustainable growth in all associated areas. Because of his investments and the company he put there to manage them, the country was able to for the first time since its independence to salvage its farmland and culture, giving jobs to tens of thousands of farmers, traditional workers and help create an exportable national identity in local products, cuisine and so forth. Perhaps there's nothing more telling about how the locals acknowledge him, when you learn that they call him "papa" (dad) and "our space man".
So, you are most definitely right that Canonical has to be a dream job, when its leadership is so well represented in terms of human and humanistic value. Although I retain my skepticism in your ability to bring any immediate material worth to the MATE project, for the reasons noted above.