I found SmartThings is OK. their V3 hub is nice and pretty cheap (about $70). I still prefer Iris. Using some user developed drivers (reading the forums help!) I have gotten most of my Iris devices working. Still can't get the Iris Wifi Smart Switches (plug in outlets) working, and I sold my V1 Iris plug/switches on ebay. I would menaove to Home Assistant, but after trying, it is a pain without moving everything to wifi devices.
SmartThings takes more 'hacking' like work to get things working, IMHO. Read their forums, you can get lots of knowlege and help there.
Home Assistant (HA), is really a DIYers environment. You can do zwave, and you can do zigbee 1.2 devices, but you must get an interface. HA is infantely more configurable, programmable, hackable, and capable, but not as 'user friendly' for those of us that want a 'plug and play' solution. (My very smart wife doesn't want to think about supporting it if am not around -- getting older gives you those thoughts a times).
I am keeping my Iris V2 hub until I can find out if we can hack it to put home assistant on it, or a Arcus update.
I like HA due to not being dependant on the cloud/net/other euphamisms for other peoples computers to work. SmartThings does seem to require the cloud/net to work. My iris hub has battery backup and I like that. Having an automatic 'network failover' to a cell signal is great. Nothing else I have found has that.
I am looking forward to seeing what ARCUS has made available when that does come about. Since Lowes is defunding it, I am wondering if completing the conversion to open source may be a volunteer effort by the current development team. But that is just me musing.