All great questions. (Full disclosure I work for PubNub)
PubNub is way more then just pub/sub (I will get to that later on). First PubNub was built as a global distributed network for a reason - so we could provide low-latency connectivity for all devices around the world through local Points of Presence. Because of this distributed architecture your devices will always connect to the POP closest to them and because we replicate messages globally, if a server or node goes down you will automatically be reconnected to the next closest node with no message lose. Because of this, PubNub provides 99.999% uptime SLA's to all customers.
When it comes to build vs. buy decision, I can tell you that many of our customers started by thinking they could build it themselves but quickly realized the undertaking was way more then they expected - read more here. Building and maintaining the client libraries, scaling the backend, 24-7 monitoring, and security are all things that you would need to have in-house expertise. If you take into consideration the upfront development costs, on-going maintenance - vs. downloading an SDK today, start coding today so you can get to market faster with a known scalable solution.
PubNub is priced based on transactions, so depending on how many devices and the level of traffic, I would bet the total cost would still be less than one full time employee. And for that you get to choose from over 70+ client SDK's, tap into a proven scalable architecture, take advantage of the security features already built in, have a full team ready and waiting to help 24/7, so you can focus on innovation and not infrastructure.
PubNub is way more then just Pub/Sub. PubNub provides not only realtime messaging, but also state management and serverless compute via their programmable netowrk. PubNub allows you to write and deploy functions in the network, in fact there are already 30+ prebuilt functions available in the PubNub BLOCKS Catalog that allow you to send SMS, emails, and more, when data changes.
PubNub has also created an open source project for building realtime dashboards called Project EON. This makes it super easy to provide realtime visualization for all of your device data.
You are also correct in stating that PubNub can be used for remote device control. And is being used today for just that by Insteon, Logitech, Samsung, Wink and many more.
If you have any additional questions, PubNub has an amazing support staff available 24/7 - email@example.com or click chat on the website.