For example a manual Subaru. Is it as simple as getting a mounting plat on the transmission with a coupler for the e-motor or is there some ecu mods needed?
Fundamentally, yes, it's no different from 2WD... because one engine is replaced by one electric motor. Of course most modern AWD systems are managed by a computer, which expects to talk to the engine controller to reduce power as required for traction control... but again, that's the same as 2WD.
Would this be different if it was automatic all wheel drive?
Yes, but again it is the same problem as with 2WD: an automatic transmission is unsuitable for use with an electric motor, because it is designed and programmed to keep the engine turning at some minimum speed or higher (by allowing slip in the torque converter), and that is unnecessary and unwanted with an electric motor. If you allow the transmission input (motor output) to stop, the transmission loses hydraulic pressure and so it can't stay in gear. Conversion of an automatic transmission to work well with an electric motor is expensive... and pointless, since the whole mess can be avoided by starting with a manual transmission (or not using the car's original transmission at all).
Alternatively for making an awd ev, is skipping the Axel and differential by having 4 smaller motors on each wheel viable or worth while?
Actually mounting motors with the wheels on the hubs is terrible for suspension performance (ride and handling), and packaging them in there is quite difficult. This is true whether the motor directly drives the wheel (which means the motor is way too big and heavy, as Duncan explained), or there is both a motor and a reduction gearbox (so the motor is smaller but there is also that gearbox to fit in).
Some trucks and buses mount the motors (with reduction gearboxes) on the axle beams (which is still bad for ride and handling but acceptable in heavy trucks and buses), but that means beam-axle suspension... not the independent suspension of a modern car such as any Subaru. It could work for a mostly low-speed off-road 4X4.
Keeping the motors on the vehicle's frame and driving through axle shafts - but with one motor per wheel - is a good design... but not done by do-it-yourself builders because they don't want to pay for four sets of motor and controller, and don't want to (or don't know how to) coordinate their management. This is the design of some prototype vehicles (which are "about to go into production"), such as the Bollinger and Rivian SUVs and pickup trucks.
No AWD production electric vehicle uses a single motor and drives all four wheels with it through a bunch of mechanical components. They all have at least separate front and rear motors. Even AWD hybrids generally don't use a traditional mechanical AWD system: they drive one axle through a hybrid transmission, and drive the other axle (usually the one at the far end of the vehicle from the engine) with just an electric motor or pair of motors (one per wheel).
And if awd really isn't the way, then it seems rwd is a common design, why?
Particularly in a do-it-yourself conversion project, the complication of doing AWD well is just too much to be worthwhile for almost any builder. Doing it by just replacing the engine with an electric motor, and keeping a simple non-electronic 4WD system, has been done by a few builders, typically with vehicles such as old Land Rovers or Suzukis. Most conversions are RWD because most of them are of old cars or sports cars, but front wheel drive conversions are not unusual, and have usually been done the same way as RWD (adapt electric motor to the car's original transmission).