Very good questions, i like them
Let’s split them up:
First is bridging difficult?
Well, a bridge take info from 1 protocol and send it like it’s supposed to in another format, how had is this? Zigbee uses 125 byte long massages, Ethernet is able to handle much more, so zigbee to Ethernet? Easy! Just take the message and send it along. Ethernet to zigbee? Easy, as long as you make your massages short enough, but, why would you even try to send long massages? In this case you have to make a smart online controller that forms the massages for you. This is straight forward to do. Bridging between Bluetooth can be a bit harder but I don’t know it’s working and I don’t touch it at the moment.
The cloud question, well, the cloud is just online storage and calculating power. I myself have web developing experience and know some languages (php, java, jquery) so I feel comfortable to say this:
Latency: Ethernet is fast, from the point of getting the request to receiving the final data takes about 0.01 second of less. Want prove? Go to google and type the letter a. right at that moment a will be send to google, will be processed and will send you a list of possibilities. It doesn’t take long, why are sites slower? Well, pictures and bad coding. Everything is recalculated and resent, the whole site, not just the part that’s needed. (for more, read about ajax)
Bandwidth cost: like I told before, we are talking about a maximum size package of 125 bytes. This is more than enough information to read out the most expensive sensor or control the most expensive 8 stage dimmer. Not enough for audio or video, but we don’t use it for that. I have my own side, any idea what I get for 10 dollar / month? 100gb of traffic, that means, well infinite request from your site. So bandwidth is no problem at all. Also the net becomes faster and ‘bigger’.
Reliability: the Ethernet is the only machine in the world that’s on for 30 years nonstop. It’s impossible to stop it, even for the president of the united states. The up time of a normal webserver is about 99% or more. There is a huge team working to do just that and make it all secure. A very good home server has an up time of about 95 %. When it stops, you’re in trouble, online everything is more save and another server can take over.
Security: easy the web is as secure as you want it to be. Paypal is online and only online secures, trust me, it is secure. When I program a site, it will cost you thousands of dollars before you can even hope to decrypt one password. An interesting fact, do you know what the most insecure part is of a secure system? The human part. On the internet you don’t have a human part, even my as super user can decrypt your passport that I myself encrypted. There is a lot of focus on online security. While your offline security will become outdated, I can just change 1 number and my security will become 100 times stronger. So, trust my it can be secure for not that much money.
Subscription cost: 100 gb, 10 kb for each user, 0.000….001 $ for each user. I earn more by advertising, is nothing when comparing it to a picture. 1 picture will use more bandwidth than the whole system for a month.
Don’t forget that the owner has only benefits on providing a more stable site, in that way he will have more users and earn more. I will always try to make my site as good as it gets. Google apps and so are good examples. Dropbox is more secure than whatever storage you can use. Updates are faster and calculation power can grow in just a second when needed.
My conclusion, online is the way to go.