In short, if you try to use Mod Merger for UHDP, RTC, COMPILATION and other mods you will notice an "OUT OF MEMORY" error in the log and roughly half the textures missing.

You get the same error if you uncook and merge manually with any tool that uses wcc_lite (xTcModKit, W3Oven, ModKit, ModKitGUI etc).

The solution is to split the uncooking operation in smaller bits so that wcc_lite can survive it. Obviously, you need the official ModKit.

As an example, here's how to unpack UHDP (the Ultra High Definition Project):

1. Create a folder named KITCHEN on your C:\ drive and drop the modUHDP folder inside (you can use different folder names, but you need to modify the ps1 file accordingly)
2. Save the uncookUHDP.ps1 to your modKit\bin\x64 folder (the folder that contains wcc_lite_exe). 
3. Right-click uncookUHDP.ps1 and select "Run with PowerShell". It takes a while. (PowerShell is part of Windows)

You will find the properly uncooked mod in the "uncooked" folder in the KITCHEN folder.

You can follow the same method for other stubborn mods. I may add ps1 files for other mods in the future. To do that, you need to first unbundle the said stubborn mod (if you use xTcModKit, select Yes for  "Unbundle Only", if you use wcc_lite directly, add "-unbundleonly" to the uncook command). You can see what should be in the mod in the unbundled folder. use the ­-targetdir= option on wcc_lite or modify the sample ps1 file witch correct relative pats for the mod you are trying to unpack.