That's due to the conversion process. Blutricity works on a different power plane and has a different scaling paradigm than the Buildcraft and IC2 networks. Just one example: a Thermal Expansion basic machine eats 2-4 MJ/t, a basic IC2 machine eats 3 EU/t, but a basic blulectric machine eats the conversion equivalent of 1 MJ/t. Everything in the system is much lower power. You can't rate the system by how well it converts into a different system... that makes about as much sense as calling a combustion engine useless because it can't properly power a mass fabricator through a converter. It's the highest tier Buildcraft engine versus the highest tier IC2 consumer... clearly the two don't fit together by any stretch of the imagination. IC2 howerver offers its own high-end energy producer which can very well run a mass fab properly.
Each system has its own base values and imposes its own scaling paradigms over its own progression tree. That's precisely why Power Converters is horribly broken and will always remain broken - it allows you to exploit the difference in scaling by setting a fixed conversion that completely disregards how the different power systems develop in relation to each other during the different gameplay phases, or even when these phases happen or how long they last.
TL;DR - You want MJ? Then build a boiler, not a blutricity solar panel.
(Not that the blulectric engine can't easily run at 24 MJ/t, three times faster than even Railcraft's highest tier, if you simply build it right...)
