I'd argue that any country which has the corresponding field marshalls and ressources to make this kind of doctrine work, should always go for it. Mobile Warfare synergizes well with offensive doctrine field marshalls and mostly boosts armor and mobile infantry.

With the maxed planning bonus applied, divisions under this doctrine have far higher stats than with any other doctrine, but you need obviously to wait for it to increase, so the needed waiting time between attacks is therefore longer. Divisions have initially weaker stats than with Superior Firepower, but have higher max planning and entrenchment. Grand Battleplan is a 'slow but steady' doctrine which synergizes well with field marshals having the defensive doctrine trait. It provides the biggest buffs for infantry and artillery overall. It can be used offensively and defensively, doesn't really desperately depend on your general traits to succeed. Superior Firepower is the one with the 'best balance'.

Therefore, literally only China should even consider going 'Mass Assault' because it mostly neither can afford basic artillery nor has the field marshalls to profit from Grand Battleplan. It keeps your your divisions weak and the reduction of infantry combat width doesn't compensate for it. Generally, 'Mass Assault' is the worst doctrine because you basically turn your manpower into a weapon can be just thrown away with this.

Hoi4 mobile warfare doctrine