Missed approach:
From an aircraft perspective - Single push TOGA switches = leisurely 2000 fpm clb, (thr toga toga) if LNAV available it will auto engage at 50’ or above (thr lnav toga). VNAV is available at 400’ baro but whether or not you can use it is up to your company.
Sounds simple, doesn't it? Except there are a bunch of other steps in the procedure, including coordination with the PM. The initial part of the published Go-Around/Missed Approach Procedure for the B777 goes something like this (with slight variations between operators):
PF: Push the TO/GA switch, call "Go-around Flaps 20".
PM: Select the flap lever to 20.
PF/PM: Verify the rotation to go-around attitude and the thrust increases.
PM: Verify the thrust is sufficient/adjust as needed.
PM: Verify a positive rate of climb and call "Positive Climb".
PF: Verify a positive rate of climb and call "Gear Up".
PM: Select the landing gear to UP.
PF: Above 400 ft radio altitude, select or verify a roll mode.
PM: Verify the missed approach altitude is set.
PF/PM: Verify the missed approach route is tracked.
ETC...
You might argue that we practice those steps all the time in the sim and that pilots should be competent at doing them correctly, and I would agree. The problem, however, is that when something totally unexpected (eg a failure) occurs at a critical moment, most pilots are startled, if only momentarily. It's a completely normal human response. Most will recover reasonably quickly and fall back upon practiced routines such as the go-around procedure, but some pilots do not. They take longer to recover, their subsequent performance suffers and they get things wrong. It is not at all unusual to see pilots get the go-around sequence wrong, or to omit some steps altogether in such situations. I have seen pilots forget to select the flaps to 20 before selecting the gear up; forget to check the thrust is set; forget to check the correct missed approach altitude is set, forget to check the correct modes are annunciated. The list goes on and there are several relatively recent accident reports that document the inappropriate responses of crews when they were confronted with something unexpected.