For example, English spelling today reflects relatedness between words and is not strictly phonemic. The above are all bound to particular languages. The number of signs makes a difference in how long you have to spend on learning the system. Japanese kids spend a large part of their early schooling memorizing Chinese characters. A good phoneme based system can be learned in a matter of hours.
Irregularities make for more time.
Kids learning English spelling have to spend a lot of effort on weird spelling and spelling conventions, as far as I know there are no Finnish spelling bees. The rigidity of the system is fairly recent. This is the point of the recent discussion of whether it helps or not to represent tones. Research like that of Russ Bernard and colleagues is important in settling the question of whether marking tone is helpful or just clutter. Traditional Chinese writing doesn't directly, but since different morphemes get different signs the tone marking is implicit.