Hi,
the -400 APU is simply not approuved for the -400.If it bursts noone knows what will happen.To get to know this you have to to testing and this costs.
Anyway the elektrical design is from -100 wich had no APU original.The disadvantage to bring it into safty design is to have another AOG Risk Item wich is now Cat D (120 Days) .APU could easyly bring it into TOP 5 AOG drivers.
Sparkies