It is quite complex. The reason I love DT so much is that they can craft really awesome music around odd time signatures like no band I've ever heard.
i don't think they really craft their songs around a time signature the way classical composers will employ a certain meter for specific reasons . I think they really just write music the way they hear it in their head and it turns out to be rhythmically complex or uneven.
the comparison i like to draw on is dave brubeck vs. bill evans. Brubeck was really innovative with his use of meter; he had a facility for odd meters that was really evident in his playing. the music never sounded like it was being pushed or pulled into a certain meter but rather could only be played with his sense of rhythm. Evans, on the other hand, had a facility for harmony . His chord structures were unique and executed flawlessly. the man seemed to be able to speak with the chords he built, they were creative and evoked very clear images/emotions/what-have-you.
Dream Theater is a great example of this dichotomy; they have a *serious* facility for rhythm. the way they organize music through time is just unlike many musicians (much like brubeck). they are not forcing weird rhythms just because they think it will be more interesting or yield a specific result, it's just how their musical minds organize musical ideas in time.
this isn't to say that they (or brubeck) lack an ear for harmony, they are extremely accomplished musicians with an extensive knowledge of harmony to draw from, however i don't think they have the same facility for it.
and of course there are exceptions all over the place in the catalogs of the artists discussed above, but for a broad strokes discussion of "how technical is dream theater" i think what i laid out is sufficient.