Tigers breaking down should not be modeled in the game.
1.)It would be annoying to have an expensive tiger, which probably has a small chance of breaking down in an hour battle actually break down. Anything that seriously depends on a dice roll is troublesome, ever played HPS Modern Campaigns and see a sure assault get poor results? It can drive one insane.
2.)Tanks breaking down was more of an operational/strategic concept. Tank breakdowns occurred from heavy traveling and took hours, the battles in Combat Mission are minutes (Except Operations). An unlikely break down (even though it can happen) will be highly frustrating and will leave the player feeling cheated since he lost do to bad luck.
3.)Even if a tank did break down, it should be remembered that battles are huge affairs, a single break down would not shift the big picture.
Tank mechanical durability should be added to the cost of the purchase. However it is alright in briefings to simply state the Tigers will not be available do to breakdowns.