The only way it worked in MechCommander was because none of the Clan forces worked together. You took them on individually in most cases, or where they only had limited light vehicle support. If the Clan units had been allowed to group together to defend against you, or even go out after you as a group, then it would have been pretty close to "unwinnable" for the player.
Picture the previous scenario of an Adder against a Stalker: the Adder has the speed to maintain the range it chooses, and there's not a whole lot that the Stalker can do about it. That means, if the Adder pilot can't get a decent movement modifier and cover, he stays at 22-23 range from the Stalker and fires cERPPC bolts at it with no return fire possible. If he or she can manage a nice run into suitable cover that keeps the Stalker pilot's odds in the "no shot" or "snowball's chance" category, then the Adder may be able to close to medium range for much higher odds to hit.....not that its odds are bad already with an inherent gunnery skill advantage, or that the odds of being hit are all that hazardous at that range while piloting a faster chassis.
The IS pilot basically has no options unless he or she can engage at closer ranges where the odds of hitting are substantially higher, closer to the center of the probability curve, and therefore less unbalanced. When the odds of hitting are 3 in 36 (11+) versus 0 in 36 (13+), or 6 in 36 (10+) versus 1 in 36 (12+), the lighter and faster unit is going to win most of the time. When the lighter unit does twice the damage when it hits, the odds are seriously lop-sided. Double heat sinks take an already unbalanced situation and keep them unbalanced even where the IS manages to close the range, in which case the Clan units are more capable of maintaining their overwhelming firepower continuously, while the already less powerful IS units overheat or reduce their weapons fire even further to limit heat buildup.
As said by previous posters, Clan zellbringen rules were intended to balance gameplay between Clan and Inner Sphere forces, but it wasn't well presented or put into a format that lent itself to tabletop scenario generation. The result in too many cases was that you either played Clan, or lost. Any campaign where Clan and Inner Sphere forces face off is going to have to contend with the basic fact that the IS forces CANNOT win if the Clan side is given the option to maneuver and use its advantages to their full extent. Intentionally crippling the Clan side in some manner is essential, otherwise it's not fun for the IS side.
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As a side note, the WWII misconception about 5-6 Sherman tanks to 1 Tiger needs to be debunked. The typical M4A1 Sherman tank's 75mm medium-velocity gun was only able to penetrate the front armor of a Tiger at short ranges, but the sides and rear were vulnerable out to medium ranges. In close quarters, the Sherman's more rapid hydraulic turret traverse allowed it to line up shots more quickly, so a "knife fight" favored the Sherman. At long ranges in the open, the Sherman had close to no chance in a 1:1 fight, but there were few such situations on the Western Front; most of the time the line of sight was broken up by tree lines, buildings, or hills, and combat was at ranges where the Sherman was only moderately outgunned. Then the British "Firefly" and US M4A3E8 "Easy 8" models of the Sherman were introduced, offering a 76mm high velocity gun that could penetrate the front armor of a Tiger at range. From that point on, the new models of Sherman had improved optics, faster turret and gun alignment systems, better visibility for increased odds of getting that important "first shot", and higher reliability over the Tiger, making them roughly equal on the field under actual combat circumstances.....in the rare cases where the Allies actually used tanks against tanks. Normally, that's what Tank Destroyers and Anti-tank guns were for, or else air strikes were called in. The German tanks were designed for "head to head" engagements, where the Allies designed their armor primarily to support infantry, with fighting other tanks as a secondary role, if necessary.