Nicehash is slightly more stable but you end up in a similar situation since the price per Mhash fluctuates in the same way. If you examine the profitability chart it's all based on historical prices:

https://www.nicehash.com/?p=calc

That's usually fine for the bigger, more established alts (since the known alts use the known algos - DaggerHashimoto, Equihash etc), but not for the alts which can potentially generate the most profit in shorter periods of time. So if a coin that uses some obscure algo gets pumped for a few days it's going to skew the chart.

The real benefit of Nicehash is that it's more accessible for people who are new to crypto mining. If you work out the numbers, mining ETH (for example) directly is almost always more profitable than using Nicehash. Nicehash is just simpler and more convenient.