This also applies to ICE. Most ICE drivetrains that do not have forced induction tend to be quite simple. Forced induction makes the drivetrain much more complex and puts a lot more strain on the drivetrain, and is one of the reasons why Toyota has stayed away from forced induction.
Is the drive train in a Tesla Model X P100d much simpler than that in a 1.5 NA Yaris? I'm not sure, and would love to be proved incorrect on this.
Yes.
Internal Combustion Engines have at a minimum a Combustion System, Ignition System, Fuel Delivery System, Exhaust System, and a Cooling System. These can vary in complexity but, apart from air cooling, any one of those systems will be more complex than an EV's electric power-train. On top of that, most of the parts of an ICE are mechanical moving parts where as most of the parts of an electric motor are stationary non-moving parts.
ICE vehicles almost universally have a transmission gear box that contains a configurable gear system to alter the gear ratio of the vehicle. All EVs on the market use a fixed gear box with a single reduction gear set.
> Is the drive train in a Tesla Model X P100d much simpler than that in a 1.5 NA Yaris?
Yes, dramatically.
This poses a problem for Toyota and others (e.g. Honda) who are known for having very reliable power-trains, not because their reliability will suffer but rather because other automaker's power-trains will get significantly better. That is assuming those other automakers don't find a way to screw up, I have no doubt some will.