I do love the Sunblood, but he is incredibly slow.
The Scar Vets are more there because they are pretty undercosted and can have excellent damage output when in a Bloodclaw.
As
@Joshua Horchler correctly stated, you want a single back breaking turn to completely blunt an opponent's momentum. The sunblood just doesn't fit this particularly well. He also can't march, so he would be lagging behind even the saurus warriors.
I do tend to agree with you on the Shadowstrike to a degree, but I was trying to keep the army with a focus on majority Seraphon. Given free reign, I really want to try out the Hammerstrike Force from the Stormcast- it's like an infintely punchier version of our Shadowstrike Starhost.
In defence of the Terradons, all they need to do is generally spit out a maximum of 3 mortal wounds with their rocks to completely get rid of the artillery crew of your choice, and this is what makes them excel in their role. They are to ensure that your Alpha Strike reaches their front line largely untroubled.
The Shadowstrike also nets you another quicksilver potion, which is also important.