Her S2 also does a lot of damage and she's relatively tough. I very often leave Siege out for a llooonnng time on maps. The only place she falls down is that she needs to be killing stuff to generate DP with her 2nd skill and that doesnt work on some maps. You can of course switch her back to her 1st skill, she's still one of the best DP generating units with it. I suppose the other flaw is that I can't see why you'd ever use her 3rd skill. Feels like you'd be better off with a dedicated DPS and giving up on the marginal DP increase.
There's also the fact that of all the unit types Vanguards are the ones you will use on nearly every singly map so you are getting more "return on investment" with her as well.
There's also the fact that of all the unit types Vanguards are the ones you will use on nearly every singly map so you are getting more "return on investment" with her as well.