There are three different timetables available. In the Dartmouth program, they spend junior year at Dartmouth, return to Colby for senior year, and then go back to Dartmouth again for year five. This is better socially than a traditional 3:2, IMHO, because it blends in with all the junior year study-abroad absences, and then everyone reunites senior year and graduates together before the fifth engineering year begins. And there’s a cohort that goes together - usually 6-8 students per year, according to the website. So I think it’s better than many 3:2 plans. But I agree, it’s hard to tell how much you can specialize, and whether you get an ABET degree at the end.
Then there are two plans with Columbia - a traditional 3:2, and a 4:2 where you finish your whole Colby degree before moving on to Columbia. This makes Study Abroad possible, and allows the full four-year experience… but then you’re on the hook for two more years for a second bachelor’s degree. That’s a lot of time and money for a double-bachelor’s when you could have gotten a BS/MS in that timeframe. I remember looking at the 4:2 on another student’s thread; in that case, they had a full ride to a top LAC, so it really made sense to consider because, why not maximize the free part of the education? But if you’re coming out of pocket for six full years, plus opportunity cost… oof.