The thrusters on the Service Module are important for the reentry.
From the first flight, which had multiple problems:
"Two software errors detected during the test, one of which prevented a planned docking with the International Space Station, could each have led to the destruction of the spacecraft, had they not been caught and corrected in time, NASA said on February 7, 2020. A joint NASA–Boeing investigation team found that "the two critical software defects were not detected ahead of flight despite multiple safeguards", according to an agency statement. "Ground intervention prevented the loss of the vehicle in both cases". Before re-entry, engineers discovered the second critical software error that affected the thruster firings needed to safely jettison the Starliner's service module. The service module software error "incorrectly translated" the jettison thruster firing sequence.[84]" (from Wikipedia
https://en.wikipedia.org/wiki/Boeing_Starliner )
During the second flight, there were similar problems with valves before launch and then:
"Two Orbital Maneuvering and Attitude Control System (OMACS) thrusters failed during the orbital insertion burn, but the spacecraft was able to compensate using the remaining OMACS thrusters with the addition of the Reaction Control System (RCS) thrusters. A couple of RCS thrusters used to maneuver Starliner also failed during docking due to low chamber pressure."
So, the problems seen in this third flight are problems that have occurred before, and Boeing has not succeeded in resolving them for several years, already. They are trying to understand what is happening in space by firing the thrusters on the Starliner at White Sands, now. At this point "NASA announced that while the Starliner is capable of returning the astronauts to Earth in the event of an emergency on the ISS,
the capsule is not approved to fly until its thruster issues are solved or at least better understood. [emphasis added]
The Service Module is required to perform adequately to achieve safe reentry. it must provide the change in velocity and control the attitude of the capsule during that change in velocity. I was not able to find a drawing or description of where the various thrusters are located on the combined craft nor which ones are malfunctioning. But, given the current and previous thruster problems, it is not unreasonable to fear additional failures during sustained additional uses - especially when they are still testing to "better understand" the cause(s?) of the failures already experienced.
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