Your question has the answer in it: because it can be denied on grounds of not being in good faith. GP post was talking about literally paying someone for marriage to obtain citizenship, literally the opposite of a good faith marriage in terms of this process.
People think it's a rubber stamp, but from what I've seen it's an invasive process where they poke around all sorts of aspects of your personal life to ascertain how 'good faith' your marriage is.
Sure, the GP was talking about marriage fraud, but I was under the impression that the post I was replying too was making a general statement that GC through marriage in general was not guaranteed. My bad for misunderstanding.
My XP is only one data point of course, but the process was not invasive at all. We had to provide some documentation (e.g. statements of shared bank account, utility bills with our names), go through a 30-ish minute interview where we had to explain how we met, how we got engage, etc and that was it. Now, maybe when the USCIS suspects fraud they start digging a little more and it gets painful.
The people I know had friends interviewed and it took quite a long time (a couple years at least) of that sort of thing for whatever reason. It still took quite a few years for even permanent residence, afaik citizenship was never even obtained.
I don't doubt that the experiences vary. But the entire point of my reply was that you can't just buy into citizenship with marriage because they will deny you if they find out you did that, ergo it is not guaranteed.
People think it's a rubber stamp, but from what I've seen it's an invasive process where they poke around all sorts of aspects of your personal life to ascertain how 'good faith' your marriage is.