Tennis swing vs golf swing ... difficulty/complexity:
I lived next door to a 4.0 Physics major my first two years in college. He knew I played tennis, and commented one day about the complexity of hitting a tennis ball (not specifically about serve ... all strokes). He said when you take into account a moving ball, and influence on trajectory (spin, etc), hitting a tennis ball was more complex than almost any other sport. I think his class had just run the numbers as a class project, or assignment.
Having played baseball until 15 (switched to tennis at 15), golf and tennis ... I would put hitting a golf ball sitting on the ground, and hitting a fastball with a bat at the top. If you measure difficulty by the amount of profanity produced, golf wins easily.
If you want a great laugh, find a bunch of high school jocks that have never played golf. Put a ball on the ground and have them try and hit it. The ball is just sitting there not moving, and they are all gifted

athletes. After 5 minutes there is 100mph swings, cussing, sweating and embarrassment. {this was a story told by one of our golf/tennis friends on a golf trip. He was a high school football player that was one of those jocks that couldn't get the ball airborne. He was our lowest handicap on our trip ... 5ish). The big ball (earth) greatly complicates golf. It 's always fun to watch a new golfer have to "play it where it lies" the first time.
I played baseball from little league to 15. When I quit, I was just getting a preview of what a real fastball pitch looked like from a couple of guys that matured early. All I can say is the guys who stand at the plate and hit MLB pitchers deserve mad props ... I can't even imagine.