Well, let me give you my perspective - I have been users of both HS/ML for 10 years+ - a couple years ago (well, maybe 3), I was going to go to one system - I thought that would be best because it was a pain managing both. I intially went with HS because I liked their ability to manage devices - their event and trigger structure I feel is more polished and easier to manage - and, there are multiple developers (a lot of updates, very active community). In ML (now Allonis), it was very challenging to emulate that capability. But, then, HS touch was a horrible experience. I couldn't get basic scenes working that were simple in ML. So, then I decided to go all-in with ML.
Well, that proved to be a challenge. I used Vera as a Z-wave controller - big mistake. Slow, difficult to manage my network, unpredictable. I have ~200 Z-Wave devices in a large setting (I now have 3 Z-Net's). And, I was frustrated with ML lockups and delays. I noticed that ML plugins would frequently stall the server - this has probably all been addressed but at that time, it was a big problem. The scenes were beautiful (I paid Dave's wife to design them), but very unpredictable response. Poor WAF.
So, while I was going through all this, HS released the Z-Net controller and Blade released is HSBL plugin (I used Krumpy's but it had some lingering issues which was another factor in me trying to find a single solution. - he stopped development).
The Z-Net is fantastic - supports the new + standard, and the tools and management are super. You can connect them together to cover large areas. It is Ethernet accessible. In HS, I have visibility of my network - know where to optimize, can configure many attributes and it is FAST. 2-way is instant. Oh, and it supports every Z-Wave device I throw at it (sensors, thermostats, valves, IO, etc.) - I have much more than lighting on my Z-Wave network and that to me is one of the best reasons for Z-Wave - its' diversity of offerings.
I have "never" had HS stall - it seems their engine is truly multi-threaded. I still don't get that in ML - if I have weather, Global Cache, Russound and Sonos plugins active in ML, I get lockups. I spent days trying to identify why but all I know now is that I have ML instant and the only way was to turn off weather (I actually use a weather plugin in HS and send those variables to ML now) and my stalls stopped. I shifted all the low-level control elements to HS including Russound and Global cache. Ever since, all stalls are gone. When I do have active session, it works great (in another thread I am discussing that issue).
So, my happy place now is to use ML for interface and control that doesn't exist in HS (Yamaha and DirectTV plugins for instance as they doesn't have a HS counterpart)
On ML, I wish the UI was a truly device driven experience - iPhone/iPad native app controls and instant no matter what. I think that would be the best asset of the software. My $0.02, there is only one Dave - and for him to try to stay current on the entire population of drivers/controllers seems to be a loosing battle and distracts from what is unique (the interface). Become a front-end and focus on the UI experience - minimize the backend and leverage the other systems out there. Allonis can be the front-end to all this madness. Even Alexia connectivity to Allonis would be huge - basically focussing on all things UI including voice interaction, app interaction, etc. I would even lean on Plex/Kodi for media management too - I mean, they are so far ahead and have a huge community - why compete with that?
Probably more that you asked but there you go!