I am testing out the ST Multi-purpose sensor for about a week now. Decent, small, lower cost, and it also reports back temperature, which may come in handy further down the room. However, I also notice that it is zeebee and not z-wave. Before I commit as I would need about dozen more (replace my old radio based contact sensors), is Zeebee an issue with other Smart Home solution like Home Assistant or HomeSeer.? Hate to doing more repurchase if I ever decide to migrate out of ST
Zigbee, not zeebee. (Zeebee is something else, but it is not used by smartthings.)
Unlike Zwave, Zigbee allows for multiple different profiles, so a hub might support Zigbee (like Control4) But still not be able to use the devices that work with smartthings. See the FAQ for more details (the topic title is a clickable link)
SmartThings supports two Zigbee profiles: ZHA 1.2 and parts of the newest profile, Zigbee 3.0. So again, some devices will work with some other hubs, but not all.
The following post lists Hubs currently on the market which support both Zwave and the zigbee profiles that smartthings supports. There aren’t many, but there are some. 
Alternative Hubs - #186 by JDRoberts
Homeseer does not support Zigbee. It is possible with home assistant, but not as easy as zwave.
Also remember that you will probably need some Zigbee Mains-powered devices to act as repeaters. (Z wave repeats only for Z wave, zigbee repeats only for Zigbee.)
The IKEA Tradfri plug-in pocket sockets make good Zigbee repeaters and cost around $10 each when purchased directly from IKEA.
I guess I am not the only night owl here 
Just curious, if I ever do decide to switch over to home assistant with a z-wave adapter, technically, couldn’t I make home assistant to talk to ST, thus ST would be the secondary? And if so, wouldn’t zigbee devices on the ST come across and be visible on Home Assistant?
The short answer is no, it just doesn’t work that way. 
Z wave allows for multiple hubs on the same network, with one primary and the others secondary. But the zigbee profiles that smartthings uses only allow for one hub per network.
While you might be able to add a smartthings hub as a secondary to a home assistant Z wave network, that would not expose any of the Zigbee devices to home assistant. And even the Z wave probably wouldn’t work the way you might imagine, the cloud portion of smartthings just tends to complicate everything.
When the SmartThings Hub is added into another Z-Wave network, it may not receive notifications from some battery-operated devices. SmartThings Technical Support does not provide assistance to consumers using the Hub with other Z-Wave controllers.
And again, even that is just zwave. Zigbee devices cannot be shared at all.
What other compact z-wave contact switches would you recommend?
Sorry, I’m not sure what you mean by “compact contact switch.“ 
can you link to an example?
Compact in term of size, as ST latest multi-purpose if pretty small
You wrote “contact switch“ did you mean to say “Contact Sensor?“
Oops, yes. Contact sensor ;( Sorry
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OK, Z wave is not as power efficient as Zigbee, which means manufacturers usually design it for bigger batteries so the batteries will still last a year or so. That’s why they are bigger.
Sensative has a patent on an ultra thin battery which allows them to make an ultra thin Contact Sensor that can be put in the door or window frame so it’s almost invisible. These are really nice devices, but they do tend to be more expensive.
They are weatherproof, can be painted, and work well with smartthings.
Aeotec makes a triangular sensor which may fit better for some use cases:
https://www.amazon.com/Aeotec-Aeon-Labs-ZW112-ZW112/dp/B01E0OMQR6/
There are also recessed door sensors from several manufacturers which get drilled into a hole in the door frame so again don’t show as much.
https://www.amazon.com/Recessed-Invisible-Security-SmartStart-Required/dp/B082PT71MM/
All of the other box type Z wave sensors are pretty similar in size, a little over 3 inches long.
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Sensative is really cool, but can the battery be charge? Given I can get the ST sensor for half the cost, it is something I have to take in consideration. Thank you for the explanation in term of why zwave sensor is so much bigger
The Sensative battery is good for ten years.
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