i think that's exactly it, it's not fundamentally a scam, but it's not as simple as 1:1 "5g in, 5g out" because there's always some formula to calculate how much carbon has been "saved" by the company selling the offsets taking whatever actions they are taking. which gives the companies selling offsets a financial motive to maximize that calculation, and in some cases does lead to outright scams.

the classic example is landowners selling offsets for the mature forests on their land - that's good! and incentivising the preservation of forests is good! but that forest already existed, and has existed for hundreds of years, so does it really make sense that some company can count it against their carbon emissions and claim to be "net zero" just because they paid a landowner to not cut it down?

it makes the whole concept of offsets less of an overall pure good than emitters simply not emitting carbon in the first place, because that is actually a very simple and clear calculation.