That said, hopefully they can find a way of doing a very generic "if this condition is checked, subtract this value from this other field" even if it means those simple adjustments won't check for contingencies.
Although, on the flip-side, such simple cases, to me, tend to be so simple that it's easier to just do the math myself than to wonder if the system is doing it right.
It's not just checking for contingencies. It's all the cascading implications. The classic examples is Barbarian Rage from PF1. Boosts to some attributes and minuses on other attributes have ripple effects throughout the character, sometimes in unexpected ways to the player. Those require the character to be fully reevaluated through the engine to properly reflect the effects everywhere.
If we're not going to things fully, then your second point about it probably being easier to just do yourself becomes reality. If we only do it partially, some users will find it "good enough" while others will decry us for doing an incomplete job. It's a no-win situation going down that path unless we can do it fully. And fully requires the engine to be utilized.