Re: round house kick about to hit your head
This is very much a question relating to style. I disagree with the assertion that a mawashi-geri is an inherently unstable position. Chaining kicks, punches, knee strikes and elbows together can actually allow you to attack from a very stable position, with the added advantage that your opponent has to back off.
You also have to remember that a skilled martial artist won't rely on just one kick or one punch, if the first misses you really need to watch for the return attack. I'm a big fan of following a round house kick with an elbow or hammer-fist; if/when the kick misses, your opponent is in a very vulnerable position for a close in strike as you continue to pivot.
Another point to consider is whether your opponent takes the approach of many people where the probe by kicking and punching multiple times, or whether they are the kind of person to more or less sit back and wait for an opening, striking quickly and powerfully to maximum effect. It is my experience that the more seasoned martial artist tends to rely on the latter.