I've come across this problem with Garmin maps as well, for example with Garmin's Topo Maroc v2 you can't route from Marrakech to Ouarzazate via Telouet as there's a broken link between the N9 just past Tizi n'Tichka and the Telouet road. So unless you spotted this when planning the route you would end up on a massive diversion.

I've always had a problem with using Mapsource to plan routes and then the Montana rerouting this when I've loaded them onto the unit, probably my tech knowledge with the system.

As a work around for me I plan a route on Mapsource and convert it to a track, that way when it loaded into the Montana its 'fixed' and won't get rerouted. This also overcomes any of those (rare) little gaps in OSM which prevent a route being calculated.

"Breaks" in the roads will likely not be visible on the screen, but exist in a mathematical sense. It results in the engine coming up with a crazy long route and even when you select waypoints on the direct route, it refuses to cooperate and keeps bactracking aroind the long way. Another problem arises if you try to plan routes that are very long.