I don't have any problem with Cannaregio at all. It's just that it is a large sestiere (section or neighborhood of Venice.) It has the largest population. Basically, it stretches from the train station tor the Rialto Bridge, two of the worst places to be. So if someone just recommends, "Cannaregio," someone can wind up staying near the train station, which is a nightmare of vendors coming up to you constantly. There are wonderful places to stay in Cannaregio, but just recommending Cannaregio is sort of like saying when visiting NYC, I recommend Manhattan. They are not the same in scale, but as in Manhattan, you can stay in wonderful places, or be in a ghetto. Cannaregio is something like that. Just going online and booking a hotel because it is in Cannaregio is risky. Castello presents much less risk. There are almost no bad neighborhoods. There are vaporetto stops everywhere. In Cannaregio if you want to go somewhere, because 8 year olds are not going to. be very happy with a lot of walking, you always have to take a vaporetto around the whole of Venice to get anywhere. I love Madonna Dell'Orto, I love the Tintorettos, but I'm not sure that will impress 8 year olds.
Castello is just more convenient. Rialto Bridge is a nightmare to visit during the day in July. However, in the evening it is an unforgettable romantic experience. But to get from there back to a hotel in the desirable places in Cannaregio means you have to go all the way around the whole of Venice, a long vaporetto ride because you'll never find your way back to your place on foot if you are not thoroughly familiar with Venice. It's a great place, but I'm thinking of what I'd recommend to someone who hasn't been to Venice 30 times, or who never lived there. It's a lot simpler to get back to Castello, rather than have to cross Venice from for example, from Piazza San Marco which is lovely in the evening, a nightmare during the day, to a hotel in Cannaregio. You are always going to get lost in Venice, but in CasteIlo it's easier to find your way there from anywhere. In Cannaregio if you are staying in one of the more isolated areas like near Madonna del Ortho, you are going to be pretty challenged to find your way back after dark because many streets are basically empty. The whole neighborhood around Madonna dell'Ortho is basically abandoned apartments. So it's a wonderful place, if you know your way around, but just saying, "Cannaregio" is not enough. It has the best and the worst of neighborhoods.