Dearth Vader is amazingly powerful even at 5 stars...
I agree that I wish his shards were more accessible and there should be more achievements to award them. Maybe one day I will have him at 6 stars.
Regardless he is very solid overall in my opinion...
His basic (with omega) has an 80% ability block chance which is incredibly useful... No resists or dodge for Jedi (awesome vs Yoda and QGJ)
Force crush wears down protection/ hp's very well and can add speed down with an omega... and it helps set up his final offensive skill
Culling blade is the highest single damage attack in the game when set up properly. It's truly amazing when it all comes together... I can't say enough about how powerful and useful it is when used properly.
Here's are some examples!
In challenges...
vs. the rancor...
In arena...
"If you only knew the power of the dark side"