I always kinda cringe when I read arguments like this one. What are you trying to say with them being "most expensive", "pure AI" and "doctrinal"? Does that mean the unit deserves infinite strength? Hell, maybe we should have Shocks oneshot every infantrysquad they encounter cause they're so damn expensive.
They are always compared with cheaper infantry, for obvious reasons.
Do you expect 1 T34 to win against 1 P4?
Then why do you expect 1 PG to fend off 1 shock?
Do you see anyone crying that 2-3 T70 can't beat 1 P4 even thou they cost more combined?
Why do you expect 2-3 grens to beat shocks?
Do 2 T34 will win with P4?
Just like 2 PGs will win against shocks.
Cost to effectiveness is pretty much the same.
Does that argument look better to you?
Because this is how the game is balanced.
Some units require more effort to counter then their counterparts on other faction.
Are they invincible? Only if you insist on attacking with units they hardcounter.