One thing I tell our developers (and POs) is that we can only tolerate 80% solutions so long as they also produce 120% solutions (or give us additional time/scope for them) at approximately the same rate.
I don't think it has to be even, but there is certainly a ratio.
Sometimes you have to let developers overengineer something or work on something a little too much so they don't go insane and take you with them. Or quit and go somewhere else.
Few things affect consistent delivery as much as bad employee retention and burnout.
