They are difficult to keep compared with many other marine animals and fish. You have to pay attention, do the water changes, be testing and observing in a way you may not have to with other creatures.
You have to spend a fair amount of money for a good tank set up with all that they need. And you have to spend some time making the tank escape-proof.
Although octos are not expensive, the food they need may prove quite expensive. You may have to go to some effort to procure the food, expecially the live food.
You octopus is an intelligent animal and needs attention - you have to spend some time interacting and playing with it. Most people find this a lot of fun.
Once you have your tank all set up, food sources established, and a tank maintenance routine in place, then you might say that it's easy to keep an octopus.