To the OP, I found the Boeing FMS OK and easy enough to learn quickly, but that was after 13 years on the Airbus, so the basic principles of using an FMS/FMC were already there.
As some have said, you always need to know what is happening to the aircraft without the FMS/FMC and it is best to regard the FMS as icing on the cake - nice to have, but not essential to fly the aircraft to an approach and landing. Things like speed changes below FL100 should be made on the FCU/MCP, (glare-shield) controls, not in the FMS.
Don't fly the 'plane with the FMS, think of the FMS as being your navigator - there to help you navigate and with flight planning and predictions, but not required for you to vector your aircraft.
For other functions though, it really does not take much effort or time for PM to update a value or tidy up the STAR on the ND that the radar vectors will bypass, or to display a radial-in line for example. It only takes PF a momentary glance to confirm the correct value has been entered on the scratch pad - about the same time as looking at the total fuel on board, which one would do occasionally. Or in your car; looking down to change the radio channel or looking at the time display while doing 70 mph.