<<. Is 'not enough real RAM to perform properly' an actual message using unified memory? >>
no you would see the use of SWAP growing, and your computer slowing down.
With separate display memory, the display memory was used only for display purposes. But we generally did not count display memory size when we cited overall memory. You described your current Mac as 32 GB, not 32 GB plus 8 GB of display memory.
Generally, that translates to LOTS of display memory when needed, but presents it own fragmentation problems. One reason we do not count display memory separately is that it is never made available for System data.
With unified memory all memory can be used for all purposes, so you have 8GB when needed for off-screen pre-draw so items 'snap into place' on the display. But when not doing that intensive off-screen work, the previously un-available empty display memory can now be used for system data.
Although you DO need to ADD to accommodate display memory, the amount you need to add is much more modest than previous high-end systems that added 8 GB just for display memory. [in my opinion] At around the 32 GB level, the amount you may need to add becomes almost negligible, unless doing complex wire-frames and shading, or full-motions pan and zoom like in games.
Lower limit: One static frame buffer for a 4K display is only three bytes per pixel, or about 12K. At 10 bits/pixel it is still under 16K. But there is lots of 'scratch paper' in use at all times, off-screen drawing intermediate objects and pre-rendering objects and Scaling (requires source and destination) that may require more.