I don't agree with this. I think the author can make the case that job hoppers aren't good employees in his experience. And maybe he can even say that job hoppers are generally bad. But human beings are quirky animals. Any statement about human behavior that contains the word "always" or "never" is almost certainly wrong.
There are definitely alternatives to 'job hopping' including consulting/contracting to get rid of the stigma while essentially doing the same thing. Alternatively you can get a job with (or start) a start-up so you can have 'measurement and leverage'.
I think it still makes a difference though. I mean consider the following two sentences:
"Job hoppers NEVER make good employees."
"Job hoppers ALMOST NEVER make good employees."
The former is much more strongly worded to the point of requiring overwhelming evidence that I feel you won't be able to find when dealing with human behavior.
I also struggle with this when I write, but I try to catch myself and avoid going down that slippery slope. You can spend the time trying to make each of your statements precise so that no one can challenge it like you've challenged this one. Alternatively, you could choose to put something strongly, trusting your audience to see your point and find your readers are more engaged and eager to discuss your ideas.
That's highly debateable. And even if it's true, it's very much situational.
I think it depends a lot on the purposes behind your writing. Are you writing to persuade people? Then perhaps you should consider the more strongly worded phrase. Are you writing to inform people (as this author does)? Then you're likely better off prioritizing accuracy. Like Einstein said: "If you are out to describe the truth, leave elegance to the tailor."
Mixing persuasive and informative writing is generally a bad idea (although informing people of what your opinion is is a bit of a grey area). At best, it convinces your reader that you're trying to persuade rather than inform and makes you lose credibility. At worst, it's unethical because it's stating opinion as fact.