I've done both and I don't believe I'd agree with making it a generalization. At best, there may be some averages and medians, but it is not a universal statement that "all salaried work is more lenient than all small business / startup work".
I've done a successful small photography business. It was way way way less stressful, more satisfying than office work, and I can only imagine how much better than e.g. Amazon warehouse work.
Salary is mainly a way to pool lots of resources and insure them for sickness, rotation, leaves, etc. It is literally the risk-averse option.
You want to compare salaried with its not-salaried version, not any salaried position vs any other non-salaried position.
Ok, but did it generate more income? How many hours did you tend to work?
Averages and medians are quite useful tools when you're analyzing something like a population.
I don't think saying that a salaried job tends to be "the most lenient way to make money" is that controversial.
Maybe you had a good run with your photography business, for many other people it is far more difficult to get a venture off the ground than to work a normal job.
Salaried work is by far the most lenient way to make money. Any other venture: a small business, a startup, an executive position, etc is way harder and more demanding.
In any case, what would be great is for the market to offer 4 day work weeks to those who wish it at the best market price, and give 5 days to the ones that want 5 days.