NSO also has a few other downsides. For employees, when you exercise an NSO, you actually have to pay tax at the time of exercise - the company withholds it and reports it as ordinary income tax (pay taxes in addition to the cost to exercise).
ISO grants can also have horrible tax implications, but you at least get a bit more flexibility as an employee -- this is also why November/December tend to have a disproportionally high number of exercises, as people get a full picture of their potential AMT liability caps.
When a company's valuation is skyrocketing and liquidation is highly likely (IPO or other M&A event), you'll often see companies offering early exercise, which can help avoid huge AMT hits, so employees are exercising when their strike price == the company's current fair market value.