There are some specific risks to the "coast FIRE" strategy that are assumed away with full retirement strategies:
1) If for some reason you are unable to work the "fun" job (e.g. health, mental health, economic depression, criminal record, caretaker role) then the stash will not be enough to support you. These issues are primary causes of bankruptcy and poverty in the U.S. today.
2) If your stash can cover your remaining expenses only because your employer covers health insurance, and then that gets taken away...
3) If wages fail to keep pace with inflation then the low-wage "fun" job might not cover as much of one's budget as originally projected. OTOH, someone retired on the 4% rule would, in theory, get a nominal stock market boost after a few years as corporations raised prices.
4) The 4% rule retiree has, as a final fallback plan, the ability to take the low-stress "fun" job for a few years if their stash reaches critical levels during a long bear market. The coast FIRE person would have to increase (double?) their wages during the same economic bad times, for example by returning to their professional career that they abandoned years ago. Which fallback is easier or more likely to work?
5) Not all lower-wage jobs are "fun" or "easy" or "low-stress". Managers still push to get every last bit of productivity out of these employees. Harassment, toxic coworkers, negative cultures, unsafe environments, etc. are very much a thing in lower-tier work. Arguably things are worse because most people in such jobs don't have the option to leave. As the person with more latitude to leave such environments, you'll be changing jobs often to avoid toxic people and conditions as they come and go. Thus, wages might be less than originally projected due to employment gaps.
6) If the plan is to break even living from wages, and to let a small stash grow big over the course of several years, that could be derailed if we have flat or negative returns for a decade (this would be a perfectly normal outcome for the stock market), and then you wait another decade for the stash to grow to retirement levels in a new, higher-cost environment. You end up working for 20+ years instead of doing a five-to-ten year drive to FIRE.
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To me, it seems like a risky gambit for the sake of escaping an unpleasant high-wage job. You could bail on the unpleasant job by making a lateral move or getting a promotion, and voila! there you are with completely different coworkers, a different boss, new culture, etc. And if that doesn't work out, just keep looking until it does. You could also put some effort into changing things, such as getting the toxic people shitcanned, advancing into management, automating or delegating unpleasant tasks, or replacing interpersonal arguments with decision procedures. Go to lunch or have an extended chat at least once a month with your foes in the organization and see if things don't change. If the hours are too long, think about the Pareto principle to identify the 20% of your work that leads to 80% of your results and ask what of the rest can be eliminated. Propose a new job description for yourself. Any of this seems easier to me than standing behind a cash register and dealing with jerks all day.