a diy can be faster

The Vado SL can reach 28 MPH so unless you're talking about breaking the law (assuming US), it's plenty fast enough. It's true it doesn't have a throttle (throttles can only go up to 20 MPH legally anyways), but you don't really need one if you make sure to downshift when you brake.

have more range

Incorrect. You can carry as many range extenders as you want.

https://www.specialized.com/pe/en/sl-range-extender-battery/p/173733

free of issues like this

Also a possible potential fire hazard. You know what you're getting when you buy from Specialized, and that's a top quality product.

also cost $3000 less

Pretty sure you're not going to get near the quality of Specialized's bikes with a DIY build. That includes everything including the frame. You get what you pay for.