"The grass is always greener on the other side."
After days of 8+ hours of intense physical labor, I pine for intellectual work. After days of 8+ hours of intense intellectual labor, I pine for physical work.
There are degrees and extremes of either. Some "easier" physical work, like, say, doing punchdowns in a winery, can be a ton of fun and not difficult at all (for a 6'3" man in good health). Coding new features into an old website, like, say, geolocation, personalization, analytics, gamification, for 28+ hours straight can also be fun, and not difficult at all (for somebody that's been doing it for 8 years).
When it's nice outside, doing physical labor outside is a blessing. When it's not nice outside (too hot, too cold, too windy), doing physical labor outside can be very painful.
Further: both physical labor and intellectual labor are intertwined. Academics can zone out just like the rest of us, performing intellectual labor in an untaxing stream of consciousness. Rowing a boat can be extremely mentally challenging. The mind and body are not completely separate.
I venture this, however -- extreme physical labor is far, far more difficult than extreme intellectual labor. Field labororers who've broken their back for decades and get paid slave wages are, imho, doing more difficult work than NASA scientists who've applied their minds for decades and get paid a very comfortable and secure government salary. The body can be broken more easily than the mind. And flexing the mind is also far more rewarded than flexing the bicep.
TLDR: neither physical nor intellectual labor is inherently more difficult than the other. But extreme physical labor is far more difficult than extreme intellectual labor.
Yes and no.
Yes, organizing information intelligently for people to quickly figure out the meaning or utility of a page, and any section or element therein, is rewarded by Search Engines.
No, just bolding the text you want to rank for isn't necessarily going to help or hurt your SEO.
Even if it were valuable, it is far less valuable than other things you should be working on that are known absolutely to help SEO -- like defining semantic relationships with HTML5 markup, say; or even better, creating "link-magnets" like useful, free web tools.