What is unique to this configuration is that the head is flat, unlike the V6. The major advantage is that you only need one valvetrain, the intake and exhaust system are much cheaper to make than in the conventional V-type engine.
The solution basically pre-dates a turbocharged era. So you have got your Golf, and you need to add more power without the benefit of forced induction. And the VR6 packaging is way shorter than the inline-6, simpler and lighter, perfect for something like the Corrado or MK3 Golf.
Of course, you can get well over 300 hp from the turbocharged 2.0-liter these days. The advantages of that happening are inline-6 can be built in many more configurations, also uses less fuel.
The 4 reasons why Engineering Explained says the VR6 is going to die out are as follows: its torque curve is not as smooth as the inline-4 turbo, it uses more fuel, it is bigger and also odder to work with.