In my view, any mod requiring PoD is automatically disqualified from being "best mod"
(I tried one or other of the incarnations of PDM - might have been APD, might have been PoD. I think I uninstalled it without even unpausing.)
That is subjective, whether you like the mod. But that is the true pros for 1821. It is the true limit of victoria 3, which is still quite an easy task to program according to the rest of the timeline
In the end, all is dependent of the point of view. You like eu4, you want that few years for eu4. I like vic2, I want that quite huge timeline for vic3.
That is another aspect. For you its rather just a few years to help play colonies for some time. In victoria 3, it means A LOT, as victoria games are faaar more detailed, and have far less historical gameplay time stuck between hearts of iron and eu.
Making an eu4 mod for that shitty 15 years or a start in 1356 has no obstacles, nor making the 0-9999 timeline, its like nothing, as eu4 lacks any depth for being limited to its timeline.
Vicky 3 would have far more depth and deeper mechanics regarding the timeline it is played in. If you want to add extra gameplay time to it, that really needs to worked on compared to eu4.
