Just marrying somebody is usually not sufficient though, and there are other requirements (put in place exactly to avoid this practice). For example you may be required to
* Being married for a number of years
* Residing with your spouse in the target country for number of years
After meeting all the requirements you need to request the citizenship to the relevant authority, and the whole process may take quite some time.
An example: if you marry an Italian citizen you may request citizenship after 2 years of marriage (3 if you're not living in Italy), but then it will take usually 3 more years for your request to be processed.
Also keep in mind that some nations don't allow double citizenship, or require special circumstances (I think to remember it is the case for Germany and Holland) so you may have to choose between the two.
There is no CAP for "green cards through marriage" so no waiting list, and the residency period to transition to citizenship is shorter (3 years) than the standard (5 years I believe).
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.
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.
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.