There are some metals that will get stronger at the weld point than the metal around it, which would then make it the strongest point on the frame.
And Tim has a point about how a carbon frame is made, gluing the joints together will create a weak point in some ways (shear being the weak side and the most important part of it). Now if anybody does do mono contruction than that goes right out the window and will achieve an incredible amount of strength.
But in the real world I'd put a properly/well constructed carbon frame right up to a steel one in terms of reliability. Now on the real long term side, like 20-40+ years, that may be a bit different, as long as the frame is never "impacted" it will be as strong as the day it was yanked out of the mold, but we all know that bikes take a beating over the course of the years.
Andrew