SDL1 gave an incredible cross-platformness to a program which I used to develop. It got compiled on Linux and Windows, Haiku , AmigaOS 3, MorphOS, Mac OS X, FreeBSD, SkyOS, Atari MiNT (Running on Atari Falcon!), even the GP2X and Caanoo handheld consoles. I suspect the SDL2 version is certainly not as mature on these platforms (I got confirmation for the Haiku one - and my own experience with the Windows one is painful)
SDL2 basically removes the support of 256-color logical screen "automatically" converted on a high-color screen/window. I've tried several implementations, but none of mine is as fast as what SDL1 was able to do. Thankfully, this doesn't seem to be what OpenXcom requires, otherwise the person who has already tried a port would have seen a big problem, long ago.
I'm bitter after wasting a lot of times troubleshooting, before finding out that a 6-month old stable 2.0.3 had a 100% systematic crash on SDL_FillRect, fixed in the sources repository though no changelog / bugtracker item reported it.
The missing feature which is a roadblock in my case is the support of resizing windows (when user drags window edge or uses maximize/restore icon). The SDL1 version worked perfectly, the SDL2 version destroys the DirectX context, and doesn't even give me any choice or moment to re-allocate and redraw the textures.