I have fed inside the tank and outside the tank. Both were very effective. I choose to feed in the tank now because it is more convenient and I see no ill effects.
Is either way wrong or bad for the snake? No

I do believe that feeding in a separate enclosure can lead to stress or a regurgitation.
Often times peoples snakes don't eat because they were told to feed in a separate enclosure. The process of moving the snake to a tub or box and then throwing a mouse in can be overwhelming for some snakes. Not all snakes will have a problem, but some will. I have never heard of a ball python not feeding because it wasn't being fed in a separate enclosure.

There are pros to both methods, but I find that the cons of the "outside the enclosure" method are worse than those of the "inside the enclosure".