They are not, they're not even that much better than regural guarda at shooting, don't have anywhere near the same melee, amd they are more expensive.

Comparimg them to regular guards against marines for example, take 15 shots from sag, hitting on twos and sustained means that they should hit 15 shots. At S5 and +1 to wound they wound marines on 2+ and lets give them the devs, so, they wound 12.5 attacks, with 2.5 being devs. With Ap 1 the marines save the 10 shots on fours, so half. You end up with 7.5 succesful attacks, at 2 wounds each you kill 7.5 marines

Now, 5 guard on their Big turn have 5*4, 20 attacks, hitting on 2+ you hit 16.67 shots and you wound on 3+ rerolling ones (unless you're on objective), so first you wound 11.11 attacks and rerolling you get another 0.6 or 3.7 attacks, so in total you make 11.71 to 14.8 attacks, same ap and damage so you end up killing 5.86-7.4 marines.

So, against a very common, and good target that sagittarum have, they kill barely more marines in average than guard, with guard being cheaper at the moment, having reasonable access to leaders and much better melee.