"Graft compatibility" is a complex topic, and it depends what your goals and needs are. For instance, some sources consider that Poncirus trifoliata is incompatible with some mandarins, and will overgrow at the union and ultimately girdle the tree; other sources consider that as this happens after 15 years, that's sufficient. Poncirus is widely used as a rootstock in many places, in particular in Japan for almost everything. It may make sense to consider what your actual goals are, rather than defaulting to grafting on Poncirus, as many citranges have across-the-board better performance in almost all settings. And if you do have specific needs in terms of soil type or disease resistance, that would definitely guide your choice. There's a lot of academic papers you can read on rootstock performance and compatibility, but the results are inconsistently reported, and vary between sites for the same kinds of rootstock. In general, many sources will tell you that Poncirus is slower growing and less vigorous. Do you care about tree size? Do you have specific goals as far as fruit quality? There are a lot of factors to take into consideration. If you just want to play around with propagation, Poncirus is a great rootstock to use because of very broad compatibility.
