Sometimes it is - you have to advanced down a narrow map. Or the terrain funnels you into a limited set of paths which are easily blocked. But sometimes it is not - you get a map wide enough that you can decoy the princess forces to one side and sneak the rest of the units through. Sometimes enough force can be applied that you only need to kill/cripple a percentage that allows you to force an opening. Some of this goes with choosing appropriate Princess settings, which are different for most scenarios (I go for longer engagement ranges and more herding than normal so they hold ground rather than wander about). And some of it is switching Princess behavior which does not happen automatically i.e. when their position has become untenable, I /kick them and have them pull off their home edge rather than fight on to the death (well, except for DCMS officers during the late Succession Wars but that's a little extra).
From my play experience - which of course could differ from others - it's the last one at least 75% of the time.
For example: my last Breakthrough fight pitted my light line lance (Hunchback, Whitworth, Panther, Enforcer, plus an attached Shadow Hawk) having to cross fifty hexes against eight light tanks, two Stingers, a Wolverine and a Clint, with an additional lance (Hermes, Wasp, Urbanmech, Spider) as reinforcements. No terrain layout or bot setting is going to make that a 'punch through' scenario, and that one's fairly typical of what I run into.
Is it beatable? Absolutely. My complaint isn't that it's too difficult. It's that the idea of the Breakthrough is one where the theoretical objective to evade/push aside the enemy is rarely what plays out, and instead the optimal solution is usually to hang back, let the bot string itself out as the OpFor charges you, and then kill/cripple every unit on the map. Actually attempting to play to the objective is frequently suicidal as 'run past' translates to 'lumber in a straight line for 10 turns while four light mechs get free range 2 backshots on you'. Even using a sacrifice unit as bait frequently loses half the force because of the weight of sheer numbers and relative speeds. There's occasions where the roll of the dice ends up with a quicker unit that can actually run and gun, but in my experience that's not even half the time unless you're playing an overall light force to begin with.
As for the rewards, the victory is good enough. Simply leaving the field in a Breakthrough scenario is a defeat which will cost strategic victory points (if they're being counted) and combined with other losses will give a bump to OpFor morale which has future consequences.
I would counter that by saying that no, it isn't, because it rarely actually gives anything towards victory. If you've got adjunct units I find they frequently suicide themselves by trying to play to the objective (usually by running full speed ahead and being swarmed exactly like I mentioned), resulting in a contract score loss that's equal what you get from winning. You get no salvage, you always lose crippled units, you frequently don't gain points towards victory, and the requirements to win are in practice higher than any other mission type.
Breakthrough (Attacker) manages to combine the worst of all factors while frequently not actually playing out the situation it claims to. It's almost as bad as Chase.
