I do think there are valid grounds for criticisms as it relates to Day 3 and Pickett's Charge. Lee's plan of operations for the operation were quiet sound, and it seems quiet clear Longstreet-due to disagreeing with the plan-failed to properly carry it out as ordered. Some years ago 67th Tigers made some great points on this:
Since he wrote that, Michael J. Armstrong of Brock University and Steven E. Sodergren of Norwich University
conducted a simulation that found that indeed, Lee's plan of operations was sound and could've worked had Longstreet devoted more resources to it:
Even accounting for the threat at South Cavalry Field, Longstreet could've plausibly fielded the necessary brigades without compromising flank security too much:
Likewise, Longstreet failed to properly oversee the conduct of the artillery barrage that preceded the attack:
Given all of this, I do think it is valid to attack Longstreet for his conduct on the third day. While the study notes Lee wouldn't have been able to exploit it in full, the loss of two Corps and the cutting of one of the two roads under his control would've forced Meade to retreat, granting a clear and favorable victory to Lee.