I just tested it (full realism),
Hurri 1 vs B-17G, 3 bursts, each about 3/4 seconds from about 110 yards (100m convergence setting).
Two engines smoking and 1 on fire...
P-51 vs B-17G, 3 bursts, 1/2 second each from about 110 yards (100m conv),
identical result.
109 F4, single cannon vs B-17G, about 20 rounds fired (two shots) from about 165 yards, one engine fell off, one on fire, aileron knocked off.
Doesn't seem weak to me. All depends from what distance you fire MGs and where you hit. I believe it would be very similar to AH in terms of knocking power if not for AH damage model.