The DPS of a seraphim ghetto would still be roughly 20% lower, for about the same cost, as other factions (slightly less mass cost, but more build time required to get 8 selens out because of roll-off time, and sera transports take longer to load). If you landed the transport before it died, and had 8 selens running around enemy territory, all hiding themselves and giving free radar, that might offset the downsides of your early seraphim ghetto not having as much DPS. Even a low-dps ghetto could still effectively kill off engineers, it just wouldn't be as good for situations where you want to fire continuously.
If we messed with the balance to make a seraphim ghetto more feasible (such as increasing cost + dps of selens) I think that would be fine. I like seraphim and i like selens so maybe I'm biased here. I wouldn't mind if they were 10% better at killing engineers even if i had to pay a little bit more for them.