I can think of multiple reasons to argue otherwise.
First and foremost is the amount of material already tried out and given at least a basic veneer of game balance for Necro - most prominently to my mind, Ash Wastes. ][munda's biggest hole from where I sit (Well, apart from scenarios) is the lack of vehicle rules. ][munda's designed to be played with a GM assigning scenarios and lack of vehicle rules is just a friggin huge hole in the GM's toolkit. Plus, in a setup like ][munda, variety is life.
Secondly, ][munda can at this time pretty safely be considered a dead project, and there's enough holes in it - not to put too fine a point on it, scenarios - to flat-out require using additional material. What's the easiest source? Necro.
Thirdly, I'd say that as it exists ][munda's single biggest fault is the same as the single biggest fault in Ash Wastes: lack of interoperability. As I see it, the '][munda is separate from Necro' and 'No Ash Wastes gangs inhive, ever ever' decisions serve mainly as a great big 'do not use' sign. These are faults to be overcome, just like the myriad balance issues in the original game addressed by NCE.
Fourth off, ][munda's 'underworld elements' warband type is different enough to a default Necro gang that I'd call them three different very closely related new gang types - mercenaries, smugglers, and pitfighting suppliers. They're a pretty similar beastie to Necro's house gangs, but not a direct match. One is a street gang, the other's a small organised crime cartel of (specify type).
And last but not least, my usual thought on this sort of matter: is there any reason you'd want them to not work together?
For my part, I'd love to some day see 'community editions' for Ash Wastes and ][munda made fully compatible and reasonably balanced with/against NCE/OCE - and the first step in that direction is for people to start using them together and as a result start getting a handle on what would need to be changed.
