I use them too and they can be very aggressive. I have a very large production style drill press so I don't see very much flex when pushing but really, as aggressive as these things are, you should not have to push hard. It is possible to flex the metal sleeve as it's very thin. It works great on end grain and I also have to agree with the assessment that it's best not to go to your finished size. The end guide is far from precise.
If you don't hold your work steady it can also grab it and pull it into the blade too....
I was actually using one last night to help with the zebrawood build I've started and they make light work of tough wood but again, I'd bring it close and then use your router for the rest.