Others can speak as to design and spec. But as to REMU and the NJICS/PSIC...
REMU may or may not fudge licenses and swap frequencies around without amendments, but when it comes to coverage they don't mess around. They don't risk public safety ever. That's their whole job. The only way Eatontown and Hoboken ever went operation for any amount of time on the NJICS was if REMU conducted, completed, and saw acceptable results in testing. In building and out. If an issue develops, they're quick to show up to fix it. If an agency elects not to continue or accept REMU's resolution, that's on them. Again, as you noted, public safety + comms = more politics.
What's interesting to me is that in other states it's the case that one system does do it all. In large cities around the country, and nightmare terrain in the boondocks throughout America. Cause let me tell ya, the Alleghenies in WV are no RF cake walk and SIRN is working there fine, including in the hollers of the New and Potomac Rivers. Baltimore, Cleveland, Detroit, Seattle, Minneanapolis, Toronto all have no issues with a single system.
So why is a system that has only had one hiccup in Union County, quicky resolved early complaints from Newark, and that has not failed most state agencies on it such a problem in the areas that are being complained about?
Seems like, again, politics to me.