Tri might only have had 18 monsters but it was incredibly well-paced in its progression to make it feel good regardless. 3U might have more "to do" but there's also an incredible amount of unnecessary fluff, including some of the shittiest subspecies of all time. Sand Barioth, Green Nargacuga and Rust Duramboros are all entirely worthless, and Jade Barroth only slightly less so. And to prop up these bad subspecious they nerf the original version, making those fights worse than they were previously.