Well, I have just tried to play like you would on my guitar and having your thumb on the bottom of the neck really restricts what you can do. The main thing is, it stops you from being able to do wide stretches properley (or any kind of stretch that goes further than a scale shape). If you had your thumb behind the neck where it should be then stretches would be much easier and more natural feeling. Another thing is bends and vibrato's have to be done with your fingers instead of your wrist and forearm when having your thumb in this position and (tbh I dont know why people say this) that is a really bad thing. Also, overtime when you want to start doing harder techniques like sweep picking then it will be 10x harder to learn than a normal player because of how restricted your hand is.
TO understand what I mean in an easier way, just take your hand now and cup your hand like you would when holding the neck. Now, put your thumb right next to your index finger like you would when you are playing further up the neck. Now try and spread your fingers and see how far you can get your little finger away from your index finger. Now, try spreading your fingers with your thumb in a position like it would be behind the neck and you will see your hand has a lot more freedom. Basically what I'm trying to say is putting your thumb behind the neck will make your hand "bigger" which will make playing a lot more comfortable.
Hope that helps.
