According to your own statements, people don't buff him with engines, but with other cards. So, the other cards are: Agitators, Ithline who can reliably buff him and Sirisa. Provided you get all the cards in hand (which is quite rare, bloody rare), you are playing 2+2+5 on three turns. That is, you put 9 points on board in three turns averaging 3 points per turn. If you get a valid target for Sirisa, then 6 points from her. Totally 15 points on board in 4 turns, with less than the acceptable bare minimum 4 points. This is assuming you get all 5 cards in your hand. So, now Sheldon is at 13 points. This is the absolute freaking best he can get with all the stars and moon aligning in your favor. This rarely ever happens. If your opponent is that lucky, he is anyhow going to win you.
Now, lets see the ugly side of it. You don't draw Sheldon and you have 2 mulligans left and you have an agitator in hand. You mulligan away a card and get Volunteer. What do you do? Keep the agitator in hand with a risk of bricking? or Mulligan him away? So, you will use him on Voluneer if the push comes to shove? What if he gets killed? You have 5 point voluteer bricked in your hand. All these s**t has happened to me when I try to use Sheldon. If you don't have last say, the opponent can have a tall unit removal or reset unit. Once, I got my 13 point Sheldon Shilard-ed and got 1 point Sheldon in my hand.
Given all the risk and most probably not getting last say if you build around Sheldon, the risks are higher. And all said and done, he is supposed to be the only good finisher ST has. So, please ST a break and let them have at least a couple of decent finishers.