Regarding the first few posts, I totally disagree. If the only time you could promote a cause was if you knew everything about that cause nothing would ever progress. From my understanding your site does not aim to be the definitive source of technical information. It aims to promote electric vehicles and conversions. It puts forward differing views and approaches on the subject. I think the more discussion the better.
It all comes down to requirements and constraints.
Cost, platform space, platform layout, acceleration, power, torque, weight, speed, flexibility, reliability, volts and amps are all variables that need to be understood to make an educated decision. These variables are quite often different for different people. Which is great because it is why we end up with so many different solutions.
Even with a DC motor in a home conversion decent results can be achieved with direct drive to a diff. But decent in terms of road car performance versus race/sports car performance are different things. In my case my launch speed is roughly halved if I launch in 4th gear versus launching in 1st gear. 4th gear (equivalent of direct drive) would be fine for road use but would not stack up in a performance situation.
Then there is the layout. In my case I wanted to run my motor in the transverse position. The only realistic options to achieve that would be to use a commercial drive unit like the Tesla or mount the motor to a FWd gearbox which has the diff built in.
The direct drive option is generally going to be a lower total weight. But the flip side is the final drive ratio is set in stone. A gearbox gives you the ability to pick a ratio that suits a given situation.
Introducing a gearbox can impact reliability. Electric motors as we all know produce great torque. So finding a gearbox to handle that torque is important. But even then a gearbox introduces a whole series of extra components which could fail or wear out. (Talk to me about cutches some time
The commercial products compensate for the lack of gears by utilising great current and voltage figures. The resulting acceleration and revs means the lack of ratios is not an issue. When space, budget or weight constraints mean the higher current or voltage is not possible a gearbox helps improve the end result.
Then there is the whole reversing capability. Goign direct drive means you need an electrically controller reverse as there is no mechanical reverse gear. Generally speaking this is acceptable for almost everyone. However in my case it is not acceptable. I run a DC brushed motor. To get decent performance I must advance the the brush timing. Applying electrical reverse would mean the brush timing would be retarded and would result in arcing damage to the brush springs etc. Most DC users accept the limitation and just drive very slowly in reverse. But in my case I need to drive fast in reverse so mechanical reverse seems to be the only answer other than switch to an AC motor.