So its 6 total engine slots, just like a SFE, but laid out differently as 2-2-2 instead of 0-6-0.
I assumed it was 2-8-2, taking the two side torso slots of an IS XL and putting them into the CT instead.
I also think cost balancing is a poor balance mechanism, not so long as folks in-universe keep pumping out XL engine 'mechs while being shy about Omnimechs 'for cost reasons.' If more players were balancing games and playing campaigns with C-Bills, it would be useful.
Putting those slots into the CT is a big, universal positive. Other possible drawbacks:
High Base Demand - Double heat for movement.
Inefficient Design - Half as many heat sinks (round down) can fit in the engine.
No Space for Details - Engine has no 'free' heat sinks.
Fragile - The 'mech needs to roll to avoid engine damage from heat and falls the same way it needs to avoid pilot damage.
EM Interference - The 'mech effectively has the EM Interference quirk, which triggers when it runs or jumps instead of when it fires a specific weapon.
Local Heat - While the 'mech's cooling system keeps overall heat low, heat by the engine is still high. If any ammo is in a location with the engine, the target number to avoid ammo explosions due to heat is 2 higher.
Control System Deficit - If the 'mech changes movement modes too quickly, it gains additional heat. When the 'mech walks, if it sprinted last turn it gains 5 heat. When the 'mech runs, if it was stationary or immobile last turn it gains 5 heat. When the 'mech sprints, if it was stationary, was immobile, or walked last turn it gains 5 heat.
Unconventional Design - The 'mech may not mount MASC, Superchargers, Coolant Pods, RISC Supercooled Myomers, TSM, or Radical Heat Sinks. An enemy weapon that causes the 'mech to gain heat cause 1 additional heat for every 3 heat they normally cause. 'Mech tasers and TSEMPs affecting the 'mech get a +2 bonus on their effect roll. 'Mech tasers used by the unit get a -2 to their feedback check. It is considered susceptible to the Centurion Weapon System.
Some of those are steep and others are complicated. But if you add tonnage, you might as well use an LFE. You can't physically add crits without defeating the purpose. Whatever in-game balancing issue mechanic has to use another mechanic: heat, equipment incompatibility, critical hits, pilot safety, etc.
One of the drawbacks above is a good starting point to address the significant advantages of halving engine weight and allowing the 'mech to survive a lost torso.
