To be honest, there are no real-world limitations...
The additional gamut in P3 is outside the natural colours you see in the reel world, so the variation between Rec709 ans P3 is very small in the real world...
The P3 additional gamut has nothing to do with gradients either. That is all down to bit depth; nothing else.
Projection brightness vs. a monitor also has nothing to do with P3 vs Rec709. That is just the way the different displays are set-up.
But, most projection environments are blacked-out rooms, while room setups with monitors tend to be illuminated in some way, so that tends to negate the actual screen brightness issues.
However, we always set monitor brightness to approx 85 to 100 nits, not the greater SMPTE 'standard' as that standard has never really been used as the older CRT displays flared badly as such levels.
So, 85 to 100 Nits has become the real world standard.
Mob Boss at Light Illusion