Regarding (multihit) single target weapons, we have a two dimensional spectrum in terms of multiplier and ATP boost. The ATP boost additively adds to the character ATP, while the multiplier multiplies the base damage by a certain amount. Even though sacrificial mechguns have the most multiplier of any weapon against a single target, sometimes the ATP boost from high ATP weapons is more substantial, enough for their H to match the damage of S from a sacrificial mech. In fact, in a lot of cases, the full combo of high ATP multihit weapons are just shy of the damage of CV, and is actually quite respectable.
When weapons like those are already almost able to match CV in power, Daylight Scar, having a balance of high multiplier and high ATP boost, deals more damage in a combo.
Higher base ATP is more favorable for sacrificial mechguns, and lower base ATP is more favorable for high ATP multihit weapons -- nothing can beat Charge Vulcan when there's level 81 Shifta, while on the other hand, when the support tech available is lower (Level 15/20 Shifta, and perhaps no Zalure since the support is busy with other things, or the enemies don't have enough DFP to be worth Zaluring), it is actually possible for high ATP multihit weapons to do more damage.
For reference, we have a scenario that is fairly favorable for sacrificial mechguns: HUcast with 30 SZ. Daylight Scar already deals more damage than Charge Vulcan in a combo for anything stronger in DFP than an Arlan (~800 DFP; the cutting point is somewhere between 700 DFP and 800 DFP, probably closer to 800, I didn't check throughly). For all other characters, they have much lower ATP usually, and the list grows. In their case, Daylight Scar often do substantially more damage in a combo.
Another factor is ATA. Some characters do not really have enough ATA to do more than NHS on a sacrificial mechgun without glitching, especially on high EVP enemies. Meanwhile, multihit weapons often have enough ATA to NHH. This is quite a penalty for the mechgun since each H from the latter already are close to the damage of S on the sacrificial mechgun. Not a good example since HUcast have FT, but with SZ 20 they do roughly 1365/2650 on Ill Gill with H/S respectively, and roughly 2130 with Jizai H. You can perhaps get a feel of the difference.
The conclusion so far is that sacrifical mechguns works off high ATP and high ATA, without one or the other, there are alternatives which may be better in certain cases.
It should be noted that notion of DPS can sometimes be misleading when comparing weapons in PSO. So far we did not mention the animation speed of mechguns versus the other melee multihit weapons -- for a full combo, mechguns are faster than all of them, one might argue that they still have more DPS. This is quite true for the case of fighting bosses, but during the stage, apart from quests where you can split spawns like PW4 10th floor or RoCT, everything spawns in waves. To be efficient, you would camp the spot an enemy spawns, finish it off with one or two combos, head over to camp the next spawn. Since you always wait, in this sense the attacking speed of a combo is often (not always, but often) negligible, depending on how much you have to do in a wave.
Another point worth noting is that despite mechguns being faster in a combo from start to end, some weapons can start their combos earlier, effectively lowering their combo length. One example is Twin Swords. If you start N just before an enemy spawns and hit the enemy with -HH, effectively you cut a portion of your combo time, and against some enemies, those four hits from -HH is sufficient (see next paragraph). Charge Vulcan can also do this, but I believe it's less effective since most of the length comes from the recovery times after the bullets are fired -- correct me if I'm wrong.
The total damage in a combo is also misleading in this case -- while Charge Vulcan, as well as Daylight Scar, deals more damage in a combo, many enemies don't need that much damage to kill. This evens out the part where high ATP multihit weapons often deal comparatively less damage -- they just need to be able to kill in a combo, or the same amount of combos. Furthermore, when fighting enemies weaker enemies in earlier areas, or enemies that are weakened by stacked gifoie, faster weapons such as Excalibur or Heaven Striker often are already sufficient to kill.
Dark Flow also deals less damage in a "combo", but since their combos are so fast and their damage are almost always sufficient, they often end up much faster than other options. L&K38 is essentially a long range DF without enough ATP to universally OHKO, and much harder to obtain with as much ATA as DF, but it is a potential option with high level Twins.
One last thing I could think of, just in case someone thinks my analysis is unfair to Charge Vulcans -- their range is something to be noted. While DF, DLS and other melee weapons can be "good" in my arguments, they have to attack up close, meaning after killing one enemy you camped, if there's another enemy you have to take out, mechguns are often better since you don't have to spend time to run to them. As a ranged weapon, mechgun bullets can also be glitched effectively in certain situations, such as when the target is on a different elevation level. They're also the overall best weapons against enemies which ignores attributes, such as E2 midbosses.
If you always play with a strong class and with Force support, Charge Vulcan is still the best in most cases. On weak classes, especially weaker forces, I think they are much more situational, although I don't have as much experience in this. Charge Vulcan is still the overall best, other weapons just allow you to optimize.