I have 4 AP906 devices using mesh.  I've just upgraded my external network connection so thought I might upgrade some areas of the WiFi; I'd expected it to be a straightforward affair but it seems that the AP906 is the end of the line compared to the other APs.

Looking at the 2nd chart on the mesh model compatibility chart page ( https://www.draytek.co.uk/products/access-points/mesh-compatibility - using an AP as the root node), it shows that if you have an AP906 as a root node, you can't use any other AP model using mesh.  I spoke to the sales team and they confirmed that is indeed the case.  It looks like I could upgrade the root node and then I'd be able to use AP906's for nodes but as I had only intended to upgrade one of the AP906's, I'd end up doubling my expenditure.
So, if you're considering the merits of each model before you buy, don't forget to look at this page.

I'm not looking to turn this into a Draytek bashing thread (tho I'm more than a little disappointed), more a "it is what it is" kind of comment.

I was a software engineer for many years and I'd assumed there'd be some kind of AP-specific shim which presented the next software layer with a common API so all the mesh devices would use the same code, but it seems not.   I'm very surprised by this and would love to know the reason.

rgds

Indeed, considering that the 906 is a fairly recent addition, so by no means an "old" model, one would hope for better, as Voldemort would say.