Just always check nutrition info, some of these that are presented as healthy are really not that great. In general, cereals are pure carbs pretty much so should limit the quantity to what you need in carbs, but they can also have a lot of fiber and be fairly efficient in terms of satiety to calorie ratio, I recently switched back to a small bowl in the morning for breakfast.
It’s generally become my opinion that unless I’m eating unsweetened muesli with a splash of milk pretty much every cereal except unsweetened cheerios is bad for me
I make Special K bars for get-togethers every once in awhile, and I sometimes get people who ask me if they're healthy. I always tell them that nothing in them is even the slightest bit healthy except the Special K itself, and even that's debatable.
In Japan, a company called Calbee makes an expensive cereal that is oat clusters mixed with grains and seeds and chunks of dried fruit. It is the best cereal I’ve ever had in my entire life. It is naturally sweet and filling, I feel amazing after eating it, and I am ruined from going back to anything American - even the “healthy” cereals.
I had Frosted Flakes (or something like it) after being on the Calbee cereal for a couple of years, and I was so disappointed by how it just tasted like mushy sugar. No substance, no flavor.