Hi. I built Anio's AT-AT about a year and a half ago with no modifications. I have had it displayed in a Hoth scene since then and it is still standing. I do understand the concern with the leg design.
Let me first say that I am completely satisfied with Anio's model and he is a master builder with many excellent models to offer; I would not hesitate to purchase instructions from him again. The actual building instructions are top-notch, near official LEGO quality in my opinion (Frousmoul did a fine job).
Now, the legs can be positioned in various stances whereby the model looks either like it is walking with the knee joints bent, or standing stationary with the knees straight. The trick is findind the right combination of legs straight/bent to balance the model properly. This is NOT a sturdy posable model like the official Lego models that have come out, but one that is a bit on the fragile side in terms of balancing right. Whenever I need to clean the dust from my scene I know that moving the beast and re-positioning it can be tricky. BUT, the model does work and looks leaps and bounds better than any official LEGO model that has been released to date. Also, it is relatively affordable to build via Bricklink. I am happy with the model but it does have some issues with the legs being flimsy or wobbly but once it has its centre of gravity worked out it will stand fine for long periods of time. It would not be a good model for younger users to play with, hence the UCS display moniker.