Certainly boiling the grains and the length of the boil will be contributing factors to less than ideal flavors. But if the biggest complaint is a yeasty flavor, then I would think you be mostly concerned with post-boil practices. Like the type of yeast, length of primary & secondary fermentation, completeness of fermentation, siphoning technique, etc.
I had yeasty beer back when I first started with the MrBeer kit and that was despite a full length boil and no grains. I'm convinced it was because of the type of yeast and the fact you could only do primary ferment. Plus, I never saw an extremely active ferment like I do now with my more conventional setup.