bought a Sigma 100-400

Check the focus shift of that copy (I'm yet to find a sigma lense without it).

I don't think there are specific issues with this lense that are unique to it and that other sigma lenses don't have.

For casual use (what I understand as casual at least) everything else won't matter. If you use it for action photography, I would also check the speed of the AF drive. The sigma lenses I've use are rather slow, and I always got issues with fast moving subjects coming towards me. I don't know this lense specifically, but even the 120-300 gave me slow af. Having an idea of the limit of the lense allows you to know when you have to stop using tracking and start prefocusing (not to mention that to subjects more or less parallel to you the lense will do just fine).

Regarding the dock, there are some interesting things that you can do, but I don't know if is worth the money first hand. I've found them second hand some times very cheap, if so then yes, at the very least for firmware updates. And if you are interested, it helps to profile and correct front/back focus (although I won't consider that a concern of casual users).

Finally, get the camera you will  be using it with and check af accuracy and speed in different light conditions. Also the difference between af modes of the camera, it is possible that at a given light situation (ie dim soft light) some mode stops working properly and then you need to switch to a different one or start doing manual overdrive.

All of these you make it in a couple of evenings at home or at the closest park. If you have someone to run in front of you even easier, otherwise doves and ducks.

Cheers