For US med schools, the attrition rate DUE TO ACADEMIC REASONS, is very low at <6% within 7yrs post-matriculation (7yrs accounts for those in combined degree programs).
Now u mentioned overwhelming pressure, if u meant academic pressure, then those r the numbers...
I don't have the stats, but if the 6% is overall rate of quitting or failing plus those who are kicked out, I think that the attrition rate on average is going to be higher for the 6 year BS/MD programs. At least for some of them. You would have to find out from the particular schools, if they would tell you.
I have no clue about BS/MD programs. For med school I think the stats when I attended were that about 5% don't complete US allo med school, and of that, only about 1.5 - 2% are for academic reasons (ie failed out). However a not insubstantial number of people at some schools will fail a major exam, a course, or even a whole year, and have to remediate, which may extend med school. Most schools give you multiple chances to fix things, so it's actually pretty hard to fail out.