You are right in a way, combos do require a lot of precision. The old saying goes practice doesn't make perfect. Perfect practice makes perfect. Unless you are practicing to become the worlds greatest combo player its wasted table time imo. Granted combos are a part of the game and deserve some practice time but I wouldn't dream of devoting the majority of practice time to them. The easiest way to get the ball into a hole is to shoot the CB into that ball and for it to go directly into the pocket, so it makes sense to me that's what should take up most practice time as far as pocketing balls go. The easiest way to get that done is by having a solid base to play from, so practice time also has to be devoted to fundamentals. To leave the easiest of pots you need to practice position also. This alone can make the game far easier than anything I've mentioned. You can be a ridiculous Potter but have poor positional skills so eventually you will snooker your self and that insane potting ability you have can't do anything to help you. But if you have world class positioning skills in theory you can be an average ball pocketer but you will leave yourself easy on most shots so it doesn't matter. This is why I believe everyone should devoted the most time to position drills if they are anything but a beginner/novice. Pocketing is thrown in there also when practicing position as a bonus.
My practice is split approximately as follows;
60% Position & Potting
10% Fundamentals....i strongly advise much, much more for this if they're a weak point for you
20% Safety
9% Banking
<1% Combos
Each to their own, but even the pros will avoid what look to be fairly easy combos if they can because its an unwanted risk of missing. But then again, all this practice means you probably win more frames than me with the old 1-9 combo.