I would assume he has made changes to get them to work together?
In any case, it won't be catastrophic. The issue is that because the main point of Dynamic UFOs is to add some randomness to the kinds of UFOs spawning, it's counter to that mod's aims to then render certain mission types with static UFO lists. In the context of another mod, that might be fine, but if you're just using Dynamic UFOs, using FTD as well would undermine the purpose of the mod.
(That's not to say that you couldn't make adjustments to FTD to make it work perfectly well with Dynamic UFOs. But in its current state it doesn't.)