BebuLamar

Neither is as powerful as the Metz 60. The high guide number is due to the zoom head.

DirtFarmer Loc: Escaped from the NYC area, back to MA

Since I was not doing the shooting with the SB900 I can't comment on how my friend was using it, but the batteries were indeed hot. They were alkaline batteries. I gave him Eneloops, which have a lower internal resistance so they don't get as hot with rapid shooting. Not fake news. Real time observation. Of course it's possible that he bought cheap alkalines in bulk and they were old stock, leading to reduced capacity.

And I haven't shot a bride with repeating flash but it is my custom to use 10 frames/second for group shots using flash and moderately high ISO. That limits the energy needed from the flash to preserve the charge so it would last over several shots. I did not do this on a continuous basis, probably only about 3-4 shots in a burst. And I did this with my SB800 or SB910 since I never had an SB900 so I can't say how it would respond to such treatment. The reason I use fast bursts for groups is because I note that maybe 10% of the time someone will blink in response to the preflash. Using the burst, the blinker has normally recovered by the 3rd shot. In the rare event that there's a late blinker, a fast burst results in photos in which the background does not change much, so Photoshop can easily be used to swap heads between shots.

The early SB900 units had a low threshold for thermal shutdown, according to posts from many people at that time (around 10 years ago). Later versions may have had that threshold adjusted upwards, but I have no direct knowledge of those details and I may be thinking of the SB910. There was a lot of variability at that time in posts, with some people stating that the SB900 would shut down at the drop of a hat and others saying they had no problems at all. I suspect much of that was due to variability in shooting styles.

According to Wikipedia, the SB900 and the SB910 are identical in power level, looks, inner mechanics and PCB design, but the SB910 had a re-adjusted thermal cut off circuit to prevent premature shutdowns. https://en.wikipedia.org/wiki/Nikon_Speedlight#SB-900_(discontinued)_and_SB-910_(discontinued)

ChristianHJensen

That one is not available anymore- you can obviously find a used one
The 64 is their current most powerful speedlite

Nikonian72 Loc: Chico CA

The small diffused secondary (fill) flash of the Nissin Di866 can be completely switched off. I have never used it.

GoofyNewfie Loc: Kansas City

From a review of the Nissin Di866:
https://www.techradar.com/reviews/cameras-and-camcorders/cameras/flashes/nissin-di866-983397/review

“It also includes a Metz style secondary sub flash and a host of other great features.”

Retired CPO Loc: Travel full time in an RV

Very good flash and way less than the obscenely expensive Nikon flash units. I've had mine for almost a year without one problem.