I've ridden a few converted bikes and a few purpose built e-bikes. In my opinion the purpose built ones are far better than any conversion I've ridden. In fact I now own a Gazelle.
In my opinion the current generation of mid drives are better than the vast majority of hub-drive motors, whether we're talking about the Bosch or Yamaha or Bafang etc... "Better" is a very slippery word. The Gazelle bikes have the Bosch mid-drive system which has torque sensing, but only puts out 250 or 350 watts and your bike cost $3 or 4 thousand dollars. Considering the moderate range and instantaneous power, while it's in line with similar bikes, I feel that it isn't a good value overall. Once bikes with this kind of performance are readily available for, say, $1200, the world will bend around them. But not before.