It is not a frontal assault. Sending your troops up the intake tunnels and catching the defenders unware. IS not in any universe a frontal assault. If you can sneak hundreds of soldiers inside the best fortified location in 500 miles you exploit that weakness before they close it up. So when the dust settles that fortification is yours.
He only sends
some of his troops that way, the bulk of them assault the top of the dam in an attempt to reach the towers and get inside. Even with the flanking maneuver via the intake pipes the Legion still takes heavy casualties during the battle that could be entirely avoided by drawing the NCR forces away from there.
There is a distressing lack of information of what Caesar was thinking when he drew up his battle plans, which is kind of annoying because it would be interesting to know why he's not confident enough in using the pipes to send his entire force that way. If he had the dam battle would be no contest, as all those NCR troops up top would have had nothing to shoot at and the towers would be blocked at the bottom by Legion squads, preventing them from reinforcing the interior defenders.
That the NCR fails to see the potential risk in leaving those pipes open if you side with the Legion is kind of weak IMO, since any officer worth his or her salt would check for things like that
first when fortifying any given location. Then again, I am of the opinion that General Oliver's failings are overplayed for the PC's benefit and tend to assume that he'd actually see this particular maneuver coming, so take that as you will.
If one sides with the Legion and hangs back during the assault (I did this myself to see what happened), the frontal attack force gets absolutely
slaughtered by NCR Veteran snipers until the triggered event where the surprise attack force shows up and swarms the deck (this happens once the PC reaches tower #3), at which point what's left of the frontal assault troops are able to close to melee range as the snipers turn their attention to the new set of targets. These get shot up in turn, however there are enough of them to enable the near-complete overrun of the NCR positions. Some of them bring heavy rifles of their own and take out the NCR snipers, which helps quite a bit.
If one abandons the topside assault force and heads into the dam, the battles there are actually pretty even although the Legion generally wins as they are much better at CQC. If the NCR troops had Riot Guns instead of Sawed-Offs, however, things would be ugly, and the Vets wielding Sequoias take a pretty good toll of Legion troops. Fortunately for the Legion the latter are relatively few in number and get overrun in short order.
Watching the interior fights is what got me wondering why Caesar didn't go all-out with the sneaky plan in the first place, as it's much more favorable in terms of casualties sustained. The first thing that jumps to mind is this was done so that Lanius could do his thing and lead the assault, but then he passes the buck to you which goes back to 'this is as it is so that the PC might handle it', which gets pretty stale by that point IMO.