What makes Castellos a great pipe?
Same thing that makes other great pipes great. Physics.
Allow me to philosophize. When Dunhill got into making pipes, he realized that there was a market going unserved - the high end "this is really perfect" market. I don't have the book in front of me but he says something like "Most people pay 50 cents for a pipe, and then complain that it's a piece of crap, because it is. We are selling pipes for 5 dollars, which is unheard of, but they're great, and people love them." To make a really good pipe, you have to do a bunch of things kind of carefully - drilling, shaping, fitting etc. This takes time. Yes, it's EXACTLY the kind of stuff you should, in theory be able to mechanize and keep a tight tolerance on. In practice... you need eyes and hands to really do it, to dial in every pipe.
But eyes and hands are expensive. It's actually easier (obviously) to manufacture low-tolerance one-size-fits all stuff and then add some kind of 10 cent miracle ingredient - an aluminum whirligig for example, something to separate your pipe from other, ostensibly lesser pipes. There was a sort of space-race in this regard, back in the day. Every pipe had a magic something or other. Dunhill never really did though - the magic was that they made these pipes with very smoothly fitted interiors - even their magic aluminum fitment was... just a plain tube.
Castello carries this tradition further - no tube, no nothing. Briar and a stem. Drilled correctly, polished out, lined up. And ... by golly... that's all there is to it. You buy good briar, you sit on it for a couple years (Castello says ten, I believe them, but 2 is probably enough to see real stabilization of the wood). You build a pipe carefully out of the wood, you line up holes, you make things smooth, and bingo, the thing is a cracking smoker! It's that simple.
It's hard enough to do that most pipe companies don't even try. I think it's quite correct to NOT call Castello a factory pipe. They are not really factory-ish in a lot of ways.
Are they all perfect exemplars of the pipe making craft? Hell no. But even the eccentric examples I have seen have mostly not been BAD. And I've seen BAD from a lot of other makers. Some makers really specialize in mediocrity. Some only occassionally rise to mediocrity. Castello seem to half way give a shit, to put it bluntly.
Yet they are not perfect pipes - the drilling can be a little off, or a little rough. The slotting in the stems is... a joke, honestly, compared to American artisan standards. Buttons a bit wonky, shaping... well, find a good one I guess. But smoking? Yeah, they work.
For the physics part... what we're talking about is getting a hot gas from point A to point B in one piece. Give it a nice, smooth ride. You do that, you got yourself a pipe.