Like the other answers given, I have 74 devices in Hubitat. However, I did my migration 100% in the opposite way. I joined my Hubitat to my existing Zwave network. All 74 of my devices are controllable from either/both SmartThings and Hubitat at the same time. The key word is ‘control’ - if I want to tell a switch to turn on or off, or automate those switches, either/both can do it. In fact that led to an amusing “failure” (amusing as in slap your forehead “OMG was I really that lost?”) in that I left a WebCore function running in ST while also coding a Rule on Hubitat in RM. I chose “a better” delay value in RM and was very frustrated when it kept shutting off “early.” Well, you already know the end to this story… halted the piston and it started working “as expected.”
For the down side to this method, all my “status only” devices, door/window sensors, motion, etc… devices that just sit there and have one job, to tell the controller something happened, they don’t tell Hubitat. How could they? When they got added to the ST ZWave network, the Hubitat wasn’t there. I had to go to each and then on Hubitat, start the Discover Devices process and then ‘kick’ the sensors into re-joining the network. Their ID number does not change in this process, it’s NOT an exclude/include cycle. They are still visible on ST. But now they send their status to Hubitat.
Let me give a more specific example… Aeon MultiSensor 6. I have a dozen of these that are used for motion and temperature. On ST there’s a DH that exposes the config registers of the Multisensor. I can set, for example, that the PIR resets after X seconds. Default is 4 mins, but for most of what I do, I want it in the 30 second range. That feature of being able to set the PIR Reset value does not (yet) exist in Hubitat’s DH. Therefore, if I want to set that value, I must use ST, but once that feature is added on Hubitat, I could set it from either. The motion and 5 other sensor values never show in Hubitat. Until I put Hubitat into Discover (Include mode) and click the button on the back of the MultiSensor (sometimes more than once to get it to go.) Almost right away, certainly within 2 mins, the status shows up in Hubitat. And most also send status to ST.
Now I can create Rules in RM that make use of the sensor values, and as mentioned, Pistons still work. My process is not better or worse than the Exclude/Include cycles everyone else is doing, it’s a lot of the same hassle… climb up, get the MultiSensor down, clicky stuff, climb up, put it back. The difference for me is that I have 4 Controllers, not one SmartThings. I have a Z-Stick and a Wink and a StaplesConnect all operating together to give me the home automation I think I need. Another example… my StaplesConnect was my first but it’s been “demoted” to being a Lutron Hub. About the only task it still does is receive Lutron Pico button pushes and via it’s rules, turn on and off stuff. I won’t give up my Pico’s!! 
(I have a Lutron Hub Pro on it’s way.) My self imposed difficulty is that I have all those Controllers and Exclude/Include 4 times WOULD be worse. I have 4 of them to overcome the local/cloud issue and to support Automation that wasn’t possible any other way (on that day.) My Z-Stick is 100% local, always has been. My StaplesConnect has always been local. So for those things that are really really annoying via the cloud, I’ve already had a means to get it local.
Bottom line, overall, Hubitat is quite good. It’s still early adopter phase to me, and therefore I will not complain about what it’s not. We’re all seeing both early bugs and far more important, fixes. Hubitat will, allow me to shut Wink, StaplesConnect and maybe my Z-Stick. I’ll end up just like “you” with two. SmartThings + Hubitat.
My Z-Stick is run via OpenRemote software that is quite a challenge to use, but once you’re over the hurdle, is really effective in the WAF arena. Via OpenRemote I’ve created fully custom IOS interface to my home. Should anyone in the house need it, they can whip out their Phone and have an interface in which everything is “up front.” Having already invested in making that, it’s going to be hard to train family to use the Hubitat interface, too many clicks, so the Z-Stick seems like it will stay. Still, 3 Controllers is better than the 4 I had, 5 now with Hubitat joining my home.
