That is mostly incorrect. Assaulting into fire is still an extremely effective tactic in a lot of circumstances, even in the era of automatic weapons. The trick is knowing when to do it.

General Pickett decided trotting half a mile up a hill to assault the entrenched enemy center, which had artillery support on the flanks was a good idea. It wasn't. However, he almost pulled it off, if it hadn't been for some reinforcements that had been relocated to the safety of the center after their heroic actions the previous day. That's right, Picketts men, made it to the union line, broke through, and ran right into the 20th Maine interrupting their rest. Right place, right time two days in a row.

The charge down little round top on the other hand, the 20th was almost out of ammunition, 1 - 2 reloads per man and they were outnumbered. The confederates assaulting up the hill were better supplied, but they were tired, and were scattered out of formation. Had the 20th stayed in place the Confederates would have had an opportunity to get organized and overwhelm them. Charging the disorganized enemy before they could consolidate was exactly the right decision.