Besides the simple fact that they're meant to calculate all factors happening right now, some calculators also slightly inflate or decrease earnings figures, which can be annoying sometimes. Some calculators also try to predict difficulty linearly or using other sorts of functions, but that's no better or worse than a calculator that simply extrapolates current earning figures.
In my experience,
Whattomine's acceptably accurate for what I've mined (ETH, XMR), and the same goes for
CryptoCompare, which is simple and also lets you set a pool fee for a more accurate earnings calculation. There's nothing of the sort on Whattomine, and it's honestly a bit surprising to me- even though pool fees are traditionally around 1% for most coins, it amounts to quite a bit over an extended period of time.
Last, don't forget that mining calculators are completely approximate in the near term and aren't meant to accurately predict the future. Use it as a guideline to determine about how profitable mining may be for you currently, and look at news and possible forks, etc to figure out if mining is worth it or not in a situation.
