I don't like the regular maintenance roll (core book, page 179). In Alien (1979) it didn't seems like they did regular maintenance, or they wouldn't have been surprised that they still were a long way home when they were awoken. In Prometheus and Covenant it seems that an android is doing maintenance during the journey, but I don't care much for ether movie.
Agree they did not wake up specifically to do regular preventative maintenance - but when they were woken up - it was time to check everything especially before a potentially rough landing...
That's what MU/TH/UR was for - monitoring and once something went bad enough - then get the crew up.
In the Alien universe, you can practically consider Ash, Bishop, David, and Walter to be mobile extensions of the shipboard computer - the computer is doing the heavy lift on monitoring and dispatching the mobile synthetic for the minor stuff - both under a set of mission guidelines installed by the same company owners.
There could be a lot of mixes of human to synthetic ratios in various crews depending on the ship owners and captain - For example, Dallas may have preferered not to work with synthetics, say, after something went wrong on the Archangel because of one (I'm making this part up).
Other ships might be highly automated (and more expensive to capitalize at first because of that) - but cheaper to operate long term ... but possibly more vulnerable to piracy because *most* synthetics are inhibited from harming humans.
And, area of operations may limit the infrastructure needed for large scale support of synthetics - the big factories and experts are in the core - not out on the rim - so when you operate even further out like the Nostromo - the cheap, expendable mark 1 "human" is probably the easier to support with frontier colonial infrastructure.
We live, as we dream -- alone. ~ Joseph Conrad