I think that learning to Kick at a really high standard (including head kicks) is probably the hardest thing to learn because you have to have really great flexibility (not eveyone has) and really great timing, Also you are vulnerable to take-downs, sweeps, low kicks, blocks and counters etc.
I know this because I spent 10 years training Freestyle Kick Boxing (both Semi-contact and Full contact).
I think low kicks are really effective and easier to master.
Alot of people would probably disagree with me, I would probably focus on developing really solid boxing then learn how to throw, low kicks, knees and elbows.
Would suggest not worrying too much about head kicks as they are low percentage techniques.
Solid basics beat flashy techniques 99% of the time.