A barrel of coal coke blocks is actually only about 590 million EU(~236 million MJ). Which sounds like a ton but isn't actually an extraordinary amount of power.
That's not to undermine the power density of such a barrel but it would take an extraordinary amount of time to get to that.
A DSU storage capacity reaches up to 2 billion coal blocks. A barrel can reach up to 4096 coal blocks, or 65536 if upgraded. That's not even close to being in range of the storage capacity of the DSU.
Also, being unable to be filled is an advantage. You can keep shoving coal coke blocks in an DSU and it'll almost never fill up.