What Yair says makes sense: without the sensitivity boost of micro-lenses (and with the high electron well fill factors of full frame type CCD's) MF sensors would be expected to have a lower minimum highlight headroom limited ISO speed (base ISO speed) than a Canon CMOS sensor with micro-lenses.
The 5D at ISO 50 is clearly "pushing", but the 2.5 stops are probably fine for many situations where there are not highlights too far above the metered mid-tone level but one is interested in holding shadow detail well below mid-tone level.
The exception that proves the rule is the 32MP Kodak KAF-31600 sensor that does have micro-lenses, and is rated at ISO 100-800, vs ISO 50-400 for Kodak sensors without micro-lenses like the KAF-39000. (These ratings given by Hasselblad-Imacon for the models that use these sensors.)