So, apparently there is a neverending supply of ubervamps, all you have to do is bleed someone onto the seal and you get another one.
This just proves that villains aren't engineers.
The First tried the Bringer plan to kill the girls. It failed. So it hopped to a completely different plan, using the ubervamp. It failed. So it hopped to yet a third plan, trying to get Andrew to kill the girls. This also failed, and as an aside proved how desperate the First is becoming, because come on, Andrew couldn't kill pigs.
Had the first put its ubervamp plan through proper QA first, it would have succeeded. They would have said "Yes, the ubervamp is very much improved over the standard issue vamp, but he's still vulnerable in the neck area, have him wear this steel throat guard."
Then the plan wouldn't have crashed so fast in beta, when Buffy beheaded the plan. She probably still would have taken it though, so the engineer would have said "Hmm, maybe I'll send two of them, since one did ok by itself, two or three might win!"
But no. The First, being a non-engineer, did not attempt to debug its ubervamp plan, but scrapped it and changed gears entirely.
Obviously, an engineer could take over the world.