It is true to some extent. It checks every 6 minutes i think. And most of the time that is true. But nicehash doesn't support every single miner and algorithm. There is custom algorithms that mine certain coins faster. Most of the top 5 coins to mine in nicehash will only be 20-30 cents difference all day long.
Now if you get a custom miner that is 30% more efficient on a single coin it is going to beat nicehash all day long.
For example I was mining ZEC on EWBF 0.3.3.b for around 3 months before it was included in nicehash. You can see details on how to manuually mine it in my guide #2. But it is now included in nicehash. I currently do not know of any that are that good out there right now so nicehash is good. But it doesn't mine every coin and algo still so you can find some coins that are more profitable. Generally they are hard to find and the profits run out once they become popular to mine.