I avoid NM now, so I can't really give you the sort of information you need. Actually my usual response to finding something is somewhat nicer than that. I find the bug and then send in a bug report + patch. I can only give you my experience with NM.
My first clash with NM was when someone complained to me they were not getting a DHCP lease. At the time I didn't even know what NM was, or that it was running, but soon found out when I used my the normal tools to debug the problem (ip, ping, dhclient, etc), and then something was undoing my changes. Things obviously changed, but the configuration files for those tools were untouched. Weird, it was like there was a ghost in the machine. Sniffing around I noticed some of the daemons had connections to dbus, which was a new development. I dumped the dbus messages, and eventually the path lead to NM. So it appeared someone had decided to redesign the entire configuration system for networking, moving away from configuration files I could look at in conjunction with man pages and make some sense of. Instead I was confronted with undocumented babble flying across a software bus. You can probably tell how impressed I was with that development.
Still, the solution was simple enough. Disable NM manager, debug the problem as I would normally, then re-enable NM. The problem was unrelated to NM - it was a bug the in firmware of a wireless router. The only issue NM caused was one of visibility - while NM was running it was impossible to tell at what point things were going wrong.
My second clash with NM was on a machine that has a broken wireless switch. Ie it had an internal wireless card that was fully functional, but because of the broken switch you could not turn it on. The owner plugged in a second wireless card, which worked perfectly when I configured it manually. Network manager refused to do anything with the second wireless card. I considered finding the bug in NM, but then visions of all those dbus interconnections rose up and I thought better of it. Any event driven daemon with its fingers in that many pies was probably a nightmare inside. So I just blacklisted the first wireless card, and NM was happy.
There were other clashes whose details escape me now, but suffices to say I have never had a machine which NM "just worked" on. There was always some situation in which it bugged out. Admittedly I tend to work on problems which have defeated others, so the machines tend to have some kink. But not the final one, which occured last weekend. It was a newly installed Ubuntu 8.10 machine. The owner wanted to use one of those 3G USB dongles described the article. To NM's credit the dongle just worked. Having proved the dongle worked we disconnected it, and asked NM to reconnect to 802.11. Do you think it would do that? Nope. Again I had not trouble doing it manually, NM wouldn't. In the end I just told the guy to reboot his machine if he wanted to change from 3G to 802.11.
So now we come to this 3G modem thing. It is great you NM guys are working on it. But what have you produced? I haven't looked, but is it a generic tool we can all use to probe these things, or some code deep within the guts of NM that only NM can use? It is not just an NM problem. Servers and openwrt boxes also have this problem, and they don't run NM.