The Necromorph form is an extremely interesting organism, and, in its own way, a poster child for the theory of Evolution.
Case-in-point, the Slasher.
The Slasher is capable of killing anything that is not better at killing than it (which, unfortunately, may be why they frequently target civilians). Anything before it is killed in a brutal, but effective, manner. It has only the tools required to do its job, a pair of scythes, a low level of inteligence, a mind apparently capable of memorizing patterns within small crevices and where these patterns lead, and surprisingly effecient tactics against human foes.
This is shown throughout the forms the Necromorph organism employs to infect i.e. dominate the food chain. The Leaper, for instance, is something of a blunt tool for bludgeoning survivors into submission and eventual infection, with little to no sentience required in doing so. As such, it does not have such sentience, and, if such a thing is required, the Leaper is rendered obsolete.
A situation in which a creature such as a Leaper would be rendered obsolete would be that of Isaac Clarke.
Isaac is, in a way, a superior life form to that of the Necromorph organism.
Isaac < Necromorph.
But, as I have stated, a Leaper would be insufficient for this task. But what would be?
A creature which can regenerate limbs, perhaps?
Thus, Ubermorph > Isaac. Isaac is a special case, and, as such, requires specialist treatment. Normally, Isaac would be able to eviscerate the Necromorph organism to extinction. However, the Necromorphs defy this process, which would take many millions of years, and turn it into a day.
Thusly, the Ubermorph is just a superior form, or, as Steve Papoutsis put it, an "evolved" Necromorph.