Disclaimer: The following is all my personal opinion, based on comparison between the Filco Majestouch (1) 104-key and Ducky DK9008S Shining that are listed in my sig.
tl;dr: I prefer my Filco of the two (excluding the difference in switch type), but thats not to say the Ducky is "bad".
The easiest way to sum it up is that it depends on your expectations. From what I can see, if a Filco is a 9.5 or 10-out-of-10 in terms of build quality then the Ducky is about a 7.5 or 8.
The actual physical build quality of the Ducky is fine. Though it may not be as good as the Filco, its definitely adequate for most intents and purposes.
In fact, I can't think of a situation where the DK9008S would have insufficient physical strength and the Filco would. Typing under oceanic pressure maybe, maybe?
The main difference in terms of build quality that I can see is that the Ducky adds a lot of features to the keyboard, but doesn't do it perfectly, whereas the Filco doesn't really add any features at all.
For example: the Ducky has a detachable cord, however there is very little room to insert/remove it (particularly for those with big fingers). This makes it difficult to use, and detracts from the portability of the keyboard, which was why the feature was added in the first place. Similarly, its very difficult to use the left and right cable routing slots, because the cable isn't given enough room to bend. Nobody complains about the Filco cable because there's nothing to complain about: no risk taken.
That all being said, there are two problems I have with my Ducky:
1) The Cherry stabilisers are gross.
2) The feet that are supposed to raise the keyboard barely raise it at all. I've had to prop mine up with some coins.
The way I see it is: If you want an extremely well-built (I'm hesitant to use the phrase "perfectly built") keyboard, without any additional features, go Filco.
If you want a solid keyboard that offers features the Filco doesn't (i.e. media keys, backlighting, detachable cable, shortcut keys for calculator etc.) then the Ducky is a perfectly fine alternative.