I'm not an expert and I'm sure others can chime in with definitive answers. I'm accustomed to using an iPad this way so I see cellular data as data (there's nothing else for me). However, I believe there are fall back channels for voice that may be separate from data. I don't get how voice could reach a tower but data could not reach the same tower but that might be frequencies or something like that (where one has a bit more distance than the other).
My experience with both is that when iPad as a phone loses cellular connection, so goes the iPhone. But my experience may not be universal. If someone chimes in saying it's not, I wonder if it ever works the other way too (iPhone loses cellular but iPad still has data). I think with LTE, data is data, so I believe the above speculation might only apply back in 4G and/or 3G and/or 2G. But someone else should clarify. All I've written with this answer could be wrong.
911 can still be dialed just like on an iPhone. The issue is local 911. For instance, if you are registered at 123 Main Street in Kansas City but have a 911 emergency in Miami, the need will likely exist to be able to tell the 911 operator where you are. I'm not sure it works differently with the iPhone or if both iPhone and/or iPad would be able to be pinged for location in an emergency. Again, someone else should chime in. My note about 911 is driven by the general rule about how VOIP doesn't work as well as landline 911. Things may have evolved since I stored that bit of knowledge.
The "extra step or two" I referenced has to do with how an iPhone is basically always connected where an iPad used as a phone may or may not be connected. For instance, sometimes I'll let the cellular service lapse when I'm not going to be traveling. Everything phone-wise still works fine as I'm pretty much always in a wifi zone. However, if I was between wifi zones and had the need to make a 911 call, I'd have to turn on cellular, buy the plan and then dial 911. With an iPhone, there's no option to avoid the cellular toll when cellular is not needed so it's always ready for that 911 call. Since AT&T rolled out that 3 month $25 plan, there's much less financial reason NOT to keep an active cellular data feed to avoid this scenario (as rare as it would be for the average Joe). By the way, if anyone's wondering about what happens if it's wifi only and someone calls? Same as when you have an iPhone turned off and don't take the call. It goes to voice mail. I get it the next time I connect and then call them back.
This is interesting because I was always under the impression that 911 calling was not possible through VOIP apps. What are you using on your iPad to dial 911 and have you actually tested it to know that it works?