Of course, it’s different for everyone and specific to each individual. Below is my take on it. The TL:DR version: emotionally devastating, otherwise quite easy.
For me, I found it similar to laying bricks. Each individual task is not all that hard, and you’re not getting stumped or start scratching your head for days. Each of these tasks can get so repetitive and mind-numbing though that the sheer boredom makes it extremely difficult to put in the required hours. Just like laying bricks, it becomes tiring/difficult from repetition and boredom, not from anything becoming overly difficult to do. I found passing certain classes, when I still had classes to take, much more challenging and difficult. Remember though, there is a LOT of work to do before completing a PhD. A LOT!!!!! That’s where the difficulty is. It can be overwhelming, and emotionally difficult, to look at a seemingly endless road of numbness. For example I always struggle when I finish planning my experiment, perform a practice run with a timer running so I can see how long it takes, and realize that you will be doing that exact procedure in the lab 150 times, which will take 600hrs to complete, or roughly 4 months of your life. Last time this happened to me it was early may, and it meant that was how my entire summer was going to be: repeating those same 4 hours again and again. Which is why when an experiment fails, depression start to sticks out it’s ugly face.
Oh, and you’re broke the whole time…. So you become a TA and correct hundreds of papers a week about things you already know just to make some money. That doesn’t help improve ones spirit. To add to this, there is little supervision, so it becomes incredibly easy to procrastinate and pseudo-work (like writing this answer right now instead of working). Since I am not under pressure to finish quickly, that only makes it easier to put things off. After I spend several unproductive days I feel guilty about myself and my work. I begin to believe I don’t have what it takes/am not PhD material. If it coincides with a failed experiment, then depression can really become hard to fight. The cherry on top is the ego in academia. There is a ton of it. One prof says to add something to a paper, another says to remove it. They love looking for anything wrong with anything, most of it banal stuff that doesn’t actually matter (often with regards to how you chose to write something), and then tear you up for it. It appears much bigger and more important in your life than it truly is. That makes you feel bad about many other small things. Reviewers from journals are sometimes even worse. I have seen, several times, someone be personally insulted by someone reading their paper. So on top of feeling like you don’t have what it takes, you also feel like you are really bad about what you do. Rarely do you get a compliment. When someone needs help and you can provide it easily, that’s your own way of feeling better about yourself, your own compliment from you to you, which isn’t as good but hopefully enough to keep your head out of the abyss.
In conclusion: constant feeling of being overwhelmed by the amount of work, feeling bad/incompetent at what you do, and importantly numb from boredom. Not hard to see why impostor syndrome and depression are so prevalent. Is the life easy? Intellectually yes. Physically yes, if you can sit behind a desk endlessly (I struggled with that part too) and have a good sleep routine. But emotionally… I found a PhD to be, and continue to be, seriously destructive.
PS: Not saying every PhD knows everything. In cases when you don’t know something (a regular occurrence) though, it does not mean it is harder, just makes for a few more bricks to lay before moving forward. For example, if there is something you don’t know you go find an answer in the literature . If it is not there, you perform an experiment to obtain the answer (forget asking a prof, that’s what they’ll end up telling you to do anyway: literature then experiment, profs have barely ever been of any help in my experience). What about equipment? If you need to learn to use equipment, you pick up a manual and read. Still in trouble, you may need to pay for a company rep to come train you for a day or find someone who uses that equipment and shadow them for a day. See? Even when you don’t know something you’re not really struggling intellectually. Other examples of normal tasks are: write a long long long paper about something you already know quite well, perform a 4 hour long extraction procedure 200 times, correcting hundreds of papers (again on a topic you are comfortable with) each week as a TA. And it is like this for years. Easy peasy!