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But you can
Well, yes, you can if you can be arsed to make copies of the factory assembly tools, but the freehub unit is always sold complete so even if you could get it apart you can't get replacements for any of the wearing parts. Compare that with most boutique hubs which, for reasons of manufacturing economy rather than design excellence, use a couple of cheap industrial cartridge bearings to spin the rotor, and insert the ratchet into the hub shell.
Have had Shimano freehubs fail on me twice in the past, and it's an annoyance you can't strip them down and service them yourself. Otherwise, they're good hubs.
Would take the DTs over Bitex though (have both at the moment). I thought that the Bitex would be the perfect hub; they are light, cheap, have bite guards, sealed bearings etc. But the pawls and pawl springs are tiny and after use over winter they were sticking/breaking. They are only cheap to replace though. The DT star ratchet has always been trouble free for me.