The initial sky island is as easy as the beginning of BotW. The initial constructs are easy to defeat (a few hits with a branch, and they’re gone).

I think by the time you jump to the surface, you’re expected to know how to fuse weapons. This will give you an advantage, and makes enemies easier to defeat. But if you don’t fuse weapons and play the game like in BotW, it becomes really hard, since enemies are designed to be attacked with stronger, fused weapons.

The depths adds another layer, as there you find resources that will make defeating enemies extremely easy: bomb flowers, muddle buds and puffshrooms are extremely useful when used strategically. As long as you avoid large enemies and bosses, the depths difficulty level is not hard. But again, you need to use fused weapons.

The main point, in my opinion, is understanding the new mechanics you can use, and play the game using them, and not like you played in BotW. It’s really a different game in a lot of aspects, specially combat.

Edit: Oh, I almost forgot: in TotK, you have the help of the sages avatars. Contrary to BotW, this game is designed to play with them, and not completely alone. A lot of people just dismiss the avatars and go alone against enemies, but this game is designed to have them around you almost always. If you’re not using them, try summoning them and having them by your side always. They help a lot.