Actual OLED TVs may produce about 800 nits in a small, full white, 2 % window of the screen. But brightness drops rapidly under 300 nits in a 50 % window and down to 150 nits with a full white screen. By this way the effect of the "Automatic Brightness Limiter (ABL)" becomes clearly visible.
By this way content with a higher level of brightness, e.g. winter sports, often looks dull in comparison to an actual LED/LCD, at least when watching TV in a moderately illuminated showroom or by daylight. UHD Premium Alliance has defined HDR requirements for OLED displays, but has missed to indicate, how intense peak levels should be depending on total screen illumination (APL). So actually 540 nits peak (upper HDR definition) can only be achieved when the major part of the screen is dark. So, to my opinion, not "peak brightness" is the major challenge for OLED displays but clear attenuation of "ABL" in brighter scenes.