Here are a couple shots from a shoot I did last week. They are sequential, one right after the other with identical settings across the board including when I converted the RAW files in Lightroom. They are very very similar and yet not identical. The one on the right is around a 1/4 stop more exposed.
This gets to the meat of the matter. What caused this minor difference? The flash unit. Now I've got a number of flashes at my place, but what happened to be on the stand when I was setting up was an Alien Bees B400. Let me say right now that I would stand by the bang for the buck factor of Paul Buffs products until the end of time. I've got 2 Alien Bees and a big White Lightning and I'd be willing to bet that any similarly priced strobe would perform similarly or worse. That said, you can see with your own eyes that you get what you pay for.
Now, a 1/4 stop or less isn't the end of the world, especially if you're shooting RAW. Just trim them a bit in Lightroom right? Well, yes for someone like me who really only works up a handful of shots from a shoot like this, but what about photographers who really need each shot to be identical? Like product photographers or still life. Places where you might be doing composites or have thousands of photos which are all going to be next to each other on some website or other. Then the idea of trying to match them all by hand doesn't look quite as quick.
And it's not just exposure that can shift. White balance of the light can shift drastically, especially when you change the power level of a flash. Drop down to lower power because you're going to shoot more wide-open to get a more shallow depth of field and all of a sudden their skin has a magenta cast. And matching skin color, especially when you don't have anything neutral in the scene to compare, can be tricky to say the least.
So what can be done? Well the sad but easy answer is to buy fancier strobes. I've got a Profoto AcuteB I use on shoots away from the studio which is very consistent in my experience. That said, it cost 6 times what the AB did. Really anal people can get the fast Broncolor and ProFoto digitally controlled packs which let you dial in power down to a 1/10th of a stop. That's control.
Is all of this a deal breaker? Certainly not for me. But it does point to the fact that there ARE differences between levels of products. Whether those differences are important to you is the question. Sometimes you do get what you pay for.