EDIT: (i have realized i have failed to update the attribute names in my commands and this may be causing the issue, but i don't know how to delete this report)

While attempting to modify and create custom mobs for a datapack i was attempting to summon mobs with modified attributes, especially MaxHealth and MovementSpeed. after summoning the mobs i discovered that they did not retain the attributes. i noticed this when i used this command: 

summon minecraft:ghast ~ ~ ~ {Health:100f,Attributes:[{Name:generic.maxHealth,Base:100}]}

only to kill the mob in 2 hits with a diamond sword. i also noticed this when trying to decrease the movement speed of a horse.

the attributes can only seem to be changed with the /attribute command, which is pretty annoying to do.