A lot of it is the difference between Disney having a dedicated in-house engineering company that, most of the time, is the best outfit of its kind in the world and Universal Creative being a relatively small and efficient office that largely relies on 3rd party contractors to do its work.
WDI is super expensive and super slow but what they produce is almost always top quality.
And sure, Hagrids might have taken Disney a lot longer to build, but assuming Disney would even build such a thing it probably a.) Wouldn't have any exposed track, b.) wouldn't be as horribly unreliable as it is and c.) Would have world-class animatronics instead of the nice, but second-rate ones it has now.
That's not necessarily a jab at Universal, but the two companies have very different ideas about what an acceptable finished attraction looks like.