XnResize doesn’t preserve or regenerate mipmaps so while this will make the warnings go away, I would presume the issues people have posted are due to not having mipmaps when the originals did have them and/or the issues I mention below.
Another issue I found is that when you change all texture formats to DXT1, DXT3, or DXT5 a lot of times textures that were meant to be black will have a yellow or green tint to them. So I always do A8R8B8G8 for textures that need to be black to prevent the tint.
I also like to keep textures maximum width or height to be 512 so if it was 1024x512 I would resize it to 512x256. Some stuff also have textures that aren’t in powers of 2 and so by resizing in bulk to a specific resolution (say 512x512) you will mess up the uvmaps and cause textures to glitch in game.
Until there is a tool that will let you intelligently resize all dds files to have a specific minimum height or width measurement (depending on the file) and just resize textures that aren’t powers of 2 to 512xSOMETHING (keeping original WxH ratio) as well as regenerating mipmaps depending on if the original dds file had them and keep the same format as the original dds files, all of these reasons above are why I prefer to resize textures manually using PaintDOTnet to make sure I don’t make any mistakes or forget mipmaps etc…