Dunno about the Boeing, but on the Bus side the recommendation is to leave FLEX unless performance is an issue - I think the main reason is the increase of thrust asymmetry and possibility of loss of control (I know, VMCA is calculated for TOGA and all that, but nevertheless, the workload increases). I can only imagine Boeing may have similar concerns.
Also, 100fpm you have cited is WAY too low on anything other than perhaps a Piper Seneca. Maybe this is some kind of sim problem, or performance calculation problem?