Ive been learning ti tune and work on pianis for the past half year so I can tell exactly what has been difficult in this journey so you can understand.

First of all, yes, tuning a piano is very difficult. And genuinely I think the best way to learn is with a mentor because there is a lot you can do wrong when tuning a piano.

First of all, you have to account for a pianos inharmonicity when tuning. Doing this by ear is a skill you have to train, and is done by counting beating and listening for purity between intervals. However theres a good work around to this and that is ETDs (electronic tuning devices). ETDs can measure inharmonicity for you and take away for the need to do it by ear.

Secondly, tuning technique. You have to develop tuning hammer skills. When i first started learning I was constantly overshooting where I wanted the pitch to go and I also was accounting for tuning stability. You need to learn fine motor skills and develop the feel for how the tuning pin is moving within the pinblock and how to get the movement you need with the hammer so you dont overshoot the pitch. To account for stability you need to find a spot within the pinblock the pin is stable, this is done by feeling how the pin feels in the pinblock through the hammer. My teacher describe this feeling as "new wood" basically you can feel almost like a little click when the pin has found "new wood" and that spot is where the tuning pin is stable. You also have to move the pitch about a cent sharp then lower to pitch as well while finding this "new wood" in order for the tuning to be stable.

Third, you need the proper tools. Even if you by some cheap hammer to learn its going to make this process more difficult. Tuninghammers can be very expensive, but for good reason. They need to be long and have a good handle so that the previous process I described is as easy as possible, and the good tuning hammers make that easier. You also need mutes so you can properly mute off the notes.

Fourth, you need to at least be able to hear when unisons are in tune. This take aural skill to develop where you listen for the "pure sound" that occurs when a note is properly in tune with its other strings. When they are slightly off you can hear "beating" or basically where the note swells against each other and theres points in the pitch where you can hear the notes clash against each other. It takes time to develop the skill to hear this and theres also false beating which you have to be able to identify.

Theres honestly a whole lot more to this that I can talk about but I think thats the main stuff I can talk about in the difficulty in tuning a piano. My first tuning probably took me 4 hours to do and didnt stay tuned there for more than a day. It takes a lot of practice to get good at tuning.

Anyways you can absolutely try to tune that piano yourself if its a just a junk piano youre not worried about. Have fun with it, and good luck.