I might be the only person alive who liked the way he tried to meld his Robot/Foundation material into a single --- can't think of the word, so "thingy" will have to do.
No, you're not the only one; I do, as well, though I will agree that there are some problems with it (even some rather serious ones). J-Sun has pointed some of them out in his post; I would also say that, in order to make it work at its optimal level, Asimov would have had to go through and recast the work to a more consistent level and tone... which would have been an enormous lot of work for rather slender returns.
As for what to call such a thing... while "superseries" is probably the closest popular thing to a name, I must admit I don't care for it much; especially as such series are not unique to Asimov here. Cabell did it with his "Biography of the Life of Manuel", which subsumed the bulkl of his writing to that time (eighteen volumes in the Storisende edition; twenty-five if one goes with the original releases, and that doesn't include a few scattered bits and pieces from other volumes as well*); Balzac did it with his
Comedie humaine (which he, too, never lived to finish, yet as it stands it would fill at least a couple of shelves on a bookcase, and covers a wide variety of types of story, not to mention various historical periods); and Moorcock is probably the most notable for such a thing, with tales of the multiverse (this ultimately includes nearly every piece of fiction he has written, which is somewhere around 100 books or better; even his lengthy Eternal Champion cycle could be said -- depending one one's frame of reference -- to be a subset of it, or another title for the entire corpus). Even Robert A. Heinlein was working toward that with his final books (see, for instance, the "People in this Memoir" section of
To Sail Beyond the Sunset, which picks up on various threads linking not only the main bulk of his science fiction, but his juveniles as well).
I'm not sure what the best term would be, really, as there are subseries within the overall arc; and each of these can even include a variety of genres as well. What it amounts to is the gradual coalescing of a unified aesthetic vision of a unique and original writer, which gives it both its fascinations and strengths, and its multitude of weaknesses and inconsistencies; but seeing such a grand vision take form and send out different branches and implications is, to me, one of the joys of literature....
*Actually, Cabell's "Biography" may cover even more genres than Balzac's "Comedie", given that it includes not only fiction but essays, plays, and a volume or two of verse as part of its structure....