I was wondering this very thing with my friend's Mya. He doesn't seem to have a problem with the taste, although I can always taste what feels/tastes like "burnt wood" on my tongue when ever I smoke from his hookah. At first I thought they were his pipes and I had just been spoiled from using my DIY hoses (which make for nice smoke).
He said he washed it often, but I'm going to have to bring over my brushes and chemicals. My chemicals essentially are a baking soda solution and concentrated lemon juice :P By boiling regular lemon juice to evaporate some of the water you can effectively lower the pH of liquid so it can clean better (at least imo). Using it in conjunction with the baking soda solution (water + baking soda) gets that fizzing action you normally get when mixing acidic and basic solutions together and it works for me to loosen the gunk from the crevices the brushes can't reach. Then just run warm water to clear it out and if you like you can run the baking soda solution once to remove the citrus smell if you would prefer, then just run some warm water again to clean it out.
Hopefully that's the problem and not his hose, otherwise we're gonna have to take a trip to Home Depot to make him a new hose