The real problem in HOI3 is that certain types of modifiers are multiplicative not additive. Within a certain category like CA bonus, it's additive so a 10% bonus from panzer leader stacks with 10% from units for 20%.
However, with other factors like leader skill, experience, weather, fort modifiers it's multiplicative.
So having a say 50% experience bonus, 50% CA bonus, 40% leader bonus for example, stacks into 1.5*1.5*1.4 = 3.15 (315% total modifier). This is quite conservative considering some of the HOI3 mods out there, and even in base HOI3 it's possible to stack up some pretty big bonuses and farm experience once you setup your OOB.
BICE takes this to an extreme, where fort modifiers are completely off the scales since there's a maximum -99% (multiplied by 0.01) for fort modifiers. However the base fort modifier is -23% per level, so a level 10 (-230% = -99%) fort has no difference from a level 5 (-115% = -99%) fort, but forces players to stack up fort bonuses to try to minimize this penalty (typically possible to achieve over 100%). This can lead to the aforementioned weird situations like in winter + mud a single brigade of MIL can hold off an entire panzer corp advancing on plains.
It also leads to weird situations like having a level 10 fort in vanilla HOI3 is literally 10x as effective as having a level 9 fort because -90% and -99% is actually a 1% vs 10% modifier (without other modifiers involved).
So in summary, balancing such huge modifiers that can multiple with other factors could be a little hard because that 50% modifier could snowball with other modifiers (if there are a lot more like HOI3). Linear scaling of bonuses can also sometimes make no sense in reality where the last 10% of effort gives you so much more value than the first 90%.
