Performing a 'data merge' to update the SuccessCount of a command block won't make the command block power a comparator.

1. Put down a command block and note its x,y,z location.
2. Put a comparator next to the command block, pointing away from it.
3. Use the command '/data merge block x y z

'
4. Notice how the comparator didn't turn on like you'd expect.
5. Use the command '/data get block x y z' and see that SuccessCount really got changed.

I think what's happening is the nbt value is getting changed by the 'data merge' command, but the block state isn't getting changed after the change is made. I can tell that by dusting the ground with redstone dust around the command block and suddenly the comparator will turn on (the dusting caused block updates next to the command block, which forced the command block to update its own block state).