This dx11 patch is probably the best way to play the game, but is absolutely garbage in terms of documentation, and actually fixing problems like the resolution crash bug above 1080p. I believe it bypasses older dx code, making the "LegacyD3DResolutionHack" unusable. The author should have realized this and fixed the problem, but no. It works only in the sense of randomly crashing in certain areas that trigger the bug, which require you to run that section in 1080p. Therefore, I run the game in a window, with fullscreen set to 1080p for those areas.

The documentation doesn't explain the settings enough, and it's placed randomly throughout the readme. You have to guess at most of them, and bump mapping will not work if you do not set values to things like ParallaxRange, which is default to 0. Parallax is different from bump mapping, but I honestly don't notice any improvement unless I enable parallax, and it doesn't enable without setting the range. Which is NOT properly documented.

FastAltTab doesn't seem to do anything, as every time you alt-tab from full screen, it pauses to reload for no legitimate reason, while windowed mode has no issue. I ended up playing in windowed mode using borderless gaming.

SSR is cool, but SSRFresnelAmount/etc is not properly documented, so I don't know what the range does. You can maybe figure out half of the settings from the documentation, and be left guessing everything else.

HDR may or may not work properly in windowed mode. If not, it will constantly re-adjust for no reason. I think I got it working, but I have no clue what causes it to bug out. edit: seems to be associated with resolution bug, when running less than native resolution in a window.

The AI upscaling pack breaks blood pool transparency, and SSR on transparency, so that's a thing. Probably should be removed from the pack. It adds nothing anyway, so I don't know why it was included. edit: This could possibly be due to losing transparency data in the conversion process. I've had positive results using XnConvert for other old games.
Also, it's pretty hilarious that Unreal Tournament supported S3TC compressed textures that you could run on a 64 Megabyte videocard, yet these new renderers don't do compression afaik, which ruins the lives of people with low memory video cards. I got 8GB, but texture compression would allow 2-3GB cards to use these texture packs, which was standard for early dx11 hardware, and the hardware shortage probably means there are plenty of people still using them. Typical issue with every game that supports texture mods, as none of them compress, and you're left with wasting VRAM, and 32-bit memory limit issues.

Texture filtering with default settings doesn't seem to work, but as soon as I turned on the other options, the mipmaps come back and require anisotropic filtering to be enabled, which is default to 0. Something is buggy here.

Color quality and lighting.... Pretty crap without HDR. The game seems to be rendering internally @ 16-bit, which is typical of old 3dfx era games, which most got patched, but this dx11 renderer doesn't seem to do good 16-32 conversion like other game patches. Also, the lighting seems more original using OneXBlending. edit: could be low quality source textures, but that's been addressed in other games, unless it's so bad nobody knows how to fix it.

AutoFOV seems to work, and you only need to manually set your resolution in the system.ini file. This means coronas work, unlike the widescreen patch. Speaking of, that patch either crashes outright, or doesn't have working corona effects. The widescreen patch only fixes the game using GLIDE or old dx, and does nothing for the dx11 renderer, which will still crash above 1080p. Therefore you don't need it for dx11, and the game looks better without it.

Vsync. Don't enable it. I think it's broken, or only works under specific conditions. Didn't work for me.

DXVK.... Works better than native dx11. So that's a thing.
Audio: Use IndirectSound. EAX with ALchemy sounds better, but muffles the hell out of the cutscene dialogue, which some of them are bad enough without echoing. One of the features (spatial?) is NOT listed in the preferences menu, and requires editing the system.ini.
Glide: ... I don't know why people would use this, outside of the dx11 issues, but Nglide is generally superior to dgVoodoo for performance and plugnplay use. dgVoodoo is generally my last resort for anything, as it doesn't really properly fix anything it claims to, especially the DX modes. The widescreen patch isn't even using up to date versions
edit: DDraw.dll from dgVoodoo crashes the game on load. If anyone has crashing issues with the widescreen patch, delete this file. This is why I don't use dgVoodoo. It doesn't ever work right. For example, the intro video creates a black background screen, but dgVoodoo's ddraw.dll breaks this black screen, much like everything else it doesn't do right.
edit2: GOG has it's own special ddraw.dll wrapper they release for other games, but not undying. I have no idea what it's based from, or why file size differs when the version # is the same. I copied the file from dungeon siege.

So anyway, you can get a nice modern experience with dx11, but there's still a bunch of issues that require fiddling.