* Some places are unreachable without Neutron Star jumps. An explorer should push the bounds, no?
Unless the Chieftain has an extreme jump range, comparable with the Asp, DBX or Anaconda - and the evidence suggests it won't, it'll be at best comparable with the Python - then that's completely irrelevant for deep edge exploration. The Corvette has 10 unrestricted internals so could carry an AFMU and every other piece of equipment an explorer might ever need, plus a few more, but it's not going to be discovering uncharted places on the galactic edges any time soon no matter how much it neutron-boosts.
It's also fine that not every exploration ship can do every exploration thing. The FDL is for most purposes an excellent combat ship ... it's not really suited to fighting Thargoids. The T-9 makes a good bulk trader ... but it can't land at outposts and its jump range is too small for rares trading. If the Chieftain is generally a great exploration ship ... but you have to choose between neutron boosting, shielded landings for SRV work, or leaving the DSS behind on any particular trip ... well, that's how the game works.
There's already a "no compromises" exploration ship - the Anaconda - so why is it so important that any/every other ship also fills that role?
* I have a life outside the game, so why would I take the slow route when I need to get somewhere?
If travel speed is the concern the Chieftain is going to be probably at least 20 LY slower (so 80 LY slower on a neutron boost) per jump than one of the conventional three anyway.
* An AFMU weighs nothing (literally), so why wouldn't you take one, even for those aimless exploration expeditions? It's not as though it has any impact on your jump range.
Well, you might, for example, have something else that you considered a more important use of that internal slot.