The problem with supporting NCs on an ISP was the file system - it required NFS, which is not very well suited to dial up connections (slow, and it resulted in effectively keeping connections open, causing nfsd to suffer badly).
We supported them at ArgoNet - but that was only because I wrote a filing system that was much better over a dial up connection (well over twice as fast for the same data). We still had to use NFS to soft-load the new filing system, but otherwise it was fine (the NFS connection was terminated as soon as possible).
Other ISPs weren't interested in supporting them because they were a nightmare to set up. We were able to sell a few "ISP kits" around the world (ISTR Sweden, Finland and Philippines).