Every inline should be reliable if the primer fires, the powder is good and there is nothing obstructing the flame channel. A flame channel might be more prone to getting obstructed if the breech plug is poorly designed, constructed or not properly maintained. Sometimes the problem can be fixed with a new breechplug, by using a better cleaning procedure between shots, or by using different powder or primers.
If you really want to try out another gun, than any type of action should be fine whether it's a break action or not.
Thompson Center guns do have a "no b.s." lifetime warranty and they will fix anything that goes wrong or at least check anything out that you send into them with a complaint.
More expensive guns should be more reliable, but just like with centerfire guns, every company can produce a lemon whether it's a break action muzzle loader or another design.
The new Knight Shadow and the Traditions Pursuit XLT break actions seem to have a good reputation for being reliable since they were chosen as new muzzle loaders of the year for 2008 by Field & Stream:
http://www.fieldandstream.com/articl...Rifles-of-2008