I am moving from SmartThings to Hubitat next month (after moving to a new home). I watched the video on Alexa integration, which demonstrates how you register devices to control them with your voice. It does not really touch on any other functionality.
For example, if I have a rule in Hubitat that is triggered when a temperature sensor passes a certain temperature threshold, can I send a message to Alexa to speak "your furnace room temperature just exceeded 90 degrees"? Can I send it to all Alexa devices? a specific Alexa device? Maybe all Alexa and Google Home devices? This is just one example.
That's pretty much all it does: exposes Hubitat devices to Alexa. Beyond that, you can also query some devices (e.g., "What is the temperature of XYZ Sensor?"). I'm not trying to make it sound boring; this is about what the SmartThings integration does, too, and it's probably how most Alexa integrations with other platforms work. It is not two-way: it doesn't expose other devices on Alexa to Hubitat, nor does it expose the Alexa device itself to Hubitat.
Yes, but you'll need something beyond Hubitat's built-in Alexa Skill integration. The only "official" starting point here would be an Alexa Routine. You should already be able to use these with SmartThings-exposed devices. The automations you can create with Alexa Routines are not as flexible as those you can create in Hubitat (or SmartThings), but you can make an Alexa device speak as part of the actions in your routine. Amazon does not have an official "TTS" API for their devices, so Hubitat is not able to make Alexa speak natively.
But that hasn't stopped some community members from figuring things out. The Alexa TTS and Echo Speaks 4 integrations are probably the most popular; you may remember earlier versions of that last one from SmartThings. The landscape should really be similar over there; this isn't a Hubitat-specific limitation. For Google devices, Hubitat has a built-in Chromecast integration that should work Google Home devices natively. It's still technically in beta, and people have mixed luck, but it's there. With either of these options, you'd be able to do the entirety of the automation on the Hubitat side, giving you more flexibility, but at the cost of some un-official-ness.
Another option is to have a Hubitat virtual Motion or Contact sensor device, exposed to Alexa, that is used as a Trigger to an Alexa Routine. So, Hubitat could detect the high temperature condition, and then momentarily change the state of a virtual contact sensor. Then Alexa would run the Routine, which would speak a phrase that you write into the Routine, and play it on whatever Alexa devices you'd like.
Thanks for adding that! That is what I intended to mention along with my first suggestion (since you need some way to trigger the routine--and note that you need a sensor for this, as contrary to the expectations of many, a "plain" switch or button exposed via Hubitat will not work).