I can only speak to the type of inflatable we had, which was a Zodiac Fastroller with an inflatable high-pressure floor and separate inflatable keel. It was only about 8 feet 9 inches in length and had a load limit of 3 passengers, but the boat interior was so small you really couldn't fit 3. The problem we had is the boat weighed about 70 pounds empty and was very hard for 1 person to lift and carry because of its bulk. The tubes on the Fastroller models, at least the ones produced around 2004, were about 2 inches wider in diameter than most other inflatable tubes. While the boat was relatively stable in the water, the wide tubes added bulk and made it very hard to get in and out of the boat. They also made the boat harder to row.
As for the inflatable floor and keel, both were separate pieces but they were connected by a tube that let air pass between them; thus, when you filled the floor you also filled the keel. Problem was, if one leaked so did the other. After a few years we noticed the floor lost pressure after several hours . We tried and tried to find the leak but never could We finally took the boat to our local Zodiac dealer and he ran into the same problem before realizing the leak was in the fitting that connected the tube between the floor and keel. He learned this because around that same time he received another boat with the same issue and that allowed him to zero in on the problem. Because Zodiac had not produced enough of these spare parts the replacement had to come from the factory in France and it took 3 months before he received it. The boat was completely useless until then. The part also cost $50 for a stupid little piece of plastic and a new ~16" piece of tubing that was of a unique diameter that looked to be roughly 1/2 inch. It seemed to us the part was about $45 overpriced but we had little choice. Even the dealer seemed disgusted, but not enough to give us a price break.
We used a Honda 2HP engine which is a great little motor. The boat made ok headway in sloppy water, but that little Honda is pretty loud when running which takes away from some of the fun. I had more fun rowing the boat, but the big tubes made it more difficult and less comfortable to row. I still deferred to rowing more often than not, but when the wind kicked up that particular inflatable was really buffeted about, whether using the motor or rowing.
The big floor in the kayak was really filled up with 2 adults. If you were going any distance you also needed a Jerry Jug of gas, mooring lines, and any necessary safety gear such as life jackets and a Type IV cushion. Once all this was in the boat there wasn't much room for anything else. The big floor filled up very quickly.
While an inflatable kayak is much thinner in width, the added length and usable storage compartments would seem to make it a bit more versatile from our perspective. It is also much easier and more efficient to paddle, and we can paddle it as a tandem when desired instead of just 1 person rowing and one acting as a passenger. Sure, the kayak can develop air and/or water leaks too, but that is a possibility with any sort of inflatable. I especially like that I can fully handle it in and out of the water by myself.
These are just my quick thoughts based on the Zodiac we had...
Rob