Are there times NOT to bid 1NT with 15 high card points and a balanced hand?
I can't speak for most experts, but I can speak for myself. I never downgrade a balanced 15-count. I occasionally upgrade a balanced 14-count, and I know several good players who do it more frequently (and announce "a good 14-17" when their partners open). Likewise, I never downgrade an 18-count into a 1NT opener, but occasionally upgrade a 17-count into a 1-level opening followed by 2NT rebid (almost always when holding a decent 5-card suit, more frequently when the suit is a major).

On the subject of 5332s, a national champion once told me that when you hold a 5-card major with 15 or 16 HCP, you should either almost always or almost never open 1NT. He was in the almost always camp; he would prefer to open the major if literally all of his high-card points were in two suits.

As to 5422 shapes, whether you should open 1NT depends on a couple of things. Most important is rebid problems. If you have 5 diamonds and 4 hearts (e.g. xx AQxx AKJxx Qx), you will have a rebid problem after partner responds 1S or 1NT. I don't think this hand is good enough to reverse, and rebidding 2D is terrible, so I would open 1NT. On the other hand, if your hand is AQxx xx Qx AKJxx, you have a good rebid after any of partner's likely responses, so there's no need to open 1NT. A difficult case is when you hold 5 clubs and 4 diamonds; if you play Crawling Stayman (i.e. that 1NT-2C-2D-2H shows that responder has a weak hand with length in both majors), you should avoid opening 1NT without at least three cards in at least one major.

Another consideration for 5422 hands is whether your values are in your long suits or your short suits. A hand like Kx Qxxxx Kxxx AK has values concentrated in the short suits; consider opening 1NT instead of 1H.