Lots of people say this, but don't listen to them. They are probably not talking from personal experience, but probably just repeating things they've heard elsewhere.
1. The push-pull might seem a bit strange the first time you use it. But after a very small amount of practice you quickly come to appreciate that it is far, far superior to a twist-ring zoom. Two reasons. One, it becomes intuitive, almost telepathic. Push the lens towards the subject too zoom in, pull it away from the subject to zoom out. You don't have to even think about it. Two, your left hand is always in the correct position under the lens to support it. You never miss a shot.
I think this myth was probably started by people who'd picked one up, thought it felt a bit strange, and didn't put in the very small amount of effort required to try it out properly.
2. The 100-400 is by far the most popular wildlife lens we have at LensesForHire, and we've had many customers take them to dusty places like Kenya. Results: No noticeable dust within any of the lenses.
I think this myth was started by people who looked at the mechanism and thought the design would mean it would probably suck dust in - i.e. a theoretical comment rather than a practical one.
