I don't know much about Slowik as an OC, but the firing seems a bit rash. Their offense wasn't as good as last year's, but the Texans have a terrible offensive line, almost as bad as the Seahawks'. Outside of Laramy Tunsil, they have no one even average on their line. In addition, all three of their WRs--Stefon Diggs, Nico Collins and Tank Dell were injured.
I guess this just goes to show you that having a terrible offensive line is the best way to get fired as an offensive coordinator. A majority of the OCs with the bottom 10 O-lines are out of a job now: Bears, Seahawks, Texans, Raiders, Patriots and Saints. The OC for the Bengals, Steelers, Giants and Titans still have their jobs.
Side note: Apparently, Demeco Ryans wanted Klint Kubiak for OC originally, so maybe now that he's available, that's where he's going?