Flex depends on the bike.
Single tube recumbents with smaller tubes, say <2.5", do bend in use, and bend an amount that is unacceptable to me. I wouldn't be concerned about weakening with bend cycles, as much as the pedaling losses. My first bent flexed a lot, and that's one of the reasons I upgraded. My current touring machine has three tubes running full length and doesn't flex at all - I'm sure it's stiffer than most DFs. I also have a monotube low racer, which has a 3.3" tube, it hardly flexes.
I sat on a couple of trikes in my LBS. They had monotubes to the crank. They both flexed less than my old 'bent, but more than I liked. Not all trikes flex that much, though.
As for empirical evidence, I follow a couple of user groups and can't remember hearing of a recumbent frame breaking. Fatigue is not the shortfall, inefficiency is.
Poor low speed handling can be an issue, but again it depends on the bike. My first one was instable below 4 mph and kinda weird above 30 as well. My current ones (single, lowracer and tandem) all handle well at both low and high speeds. DFs are still better at low speed, but mine are ok down to walking speed. I'm happy with them. And in the balance, I'd rather suffer a slower climbing speed, and reap the benefit of a better bike to fight headwinds.
I'm not evangelizing recumbents, whatever works for you is fine. Just clarifying some points you made.
