People keep saying it has less internals as the Type 6. Well, I wanted to make the best ship that could transport 75t of cargo, so I went to coriolis.io and fiddled around with all ships that could carry 75t of cargo without going over my cost limit (my design goal was keeping rebuy costs down). Comparing the keelback and the type 6, I put 5A shields into the type 6 but only 3A shields into the keelback, and the keelback ended with more shields. It was also slightly better shielded and a bit faster than the Cobra MkIV. So "less internals" is not strictly speaking correct. In that configuration, I was able to build a better ship with the keelback than I would have been able with the baseline type 6.
So I bought a keelback. And I like it so far.
It really depends on what you are going for. The standard Type-6 has more compartments overall (7 to 6), and more of them are large (5 5 4 4 3 2 2 vs. 5 5 3 3 2 2). For an exploration build, for example, this means I can hold 80 tons of cargo in a Type-6 without refitting it between tasks (64 tons if I want an SRV), whereas the Keelback is cut down to 40 tons (32 tons if using an SRV). That is a build with a discovery and surface scanner, a fuel scoop, and a size 3 shield generator. The T6 is easier to evade interdictions with and has a bit of a better jump range (3-4 Ly, but that is not a big deal if you are not in a rush). Not sure many people do exploration with either of these ships, but that is the niche mine found.
Generally, if you want versatility in loadout, then number of compartments is more important than their size, and if you want unabated hauling capacity, bigger is better even if there are fewer. Strictly speaking, the Type-6 has more of both, but the hardiness of the Keelback can be a blessing that cannot be accounted for by compartment space. It also has a bigger power plant, which definitely adds to its shield capabilities. The T6 has to run (usually), the Keelback stands and fights.
I flew a Keelback a couple of times, and I was mostly disappointed not by the internals but by the speed and agility. I am curious how much the speed can be compensated by engineers. I also was a little bothered by the rotating engine thing, but that is an argument just about everybody makes and it is not that big of a deal to me.