He's not saying NJ's or SP's are 'bad' in the sense of having worse traits or being worse personalities than the others.
He's saying NJ's maybe don't fit all of the stereotypical 'J' traits that tend to be on the J/P descriptive lists, so may appear to be more 'P' than SJ's, and the SP's maybe don't fit all of the stereotypical 'P' traits, so maybe appear to be more 'J' as compared to NP's.
"...On and on and on and on he strode, far out over the sands, singing wildly to the sea, crying to greet the advent of the life that had cried to him." - James Joyce
Well, no. SJs can be very "bouncy" with their decisions, if you examine them over a longer time-scale - even if they seem to be very prepared on shorter ones. NJs may be more messy, but their decisions are rarely "bouncy".