Difficult is relative.
Some people thing changing a tire is difficult and should only be done by a professional.
Others think overhauling a transmission is a routine job.
In newer cars it can get fairly complex. Integrating a new head unit with the factory electrical system can be a hassle because some HU's are a centerpiece of the car's "brain".
Also, a lot of cars have external amplifiers that can cause some confusion.
Routing wires can be an obstacle.
Doing a full install for a first timer would be quite a challenge and very time consuming for most people IMO.
You need to do a lot of research before you pull your first panel. You need to know what you're up against. What, if any special accessories you'll need.
I've been tinkering in car audio for well over 20 yrs now. I've been running basically the same system for around 10 yrs and I've transplanted it 3 different times. One was pretty easy, two were a bit more involved. All of them took several hours. The worst one, which was the most recent took probably 2 or 3 hrs of research and I would guess 10 hrs to install.
I'm no expert, but I consider myself a decent shade tree car audio tech.
It depends on the type of car that you do it on. Every car is different. I have no idea what kind of car you have. Overall, most cars are easy. Car audio should only be installed by someone who knows what they're doing. I'd suggest having some knowledge in the car audio field before you start blowing fuses.
You also need to have the proper size tools for the installation. I would also recommend having a tool that is powerful enough to cut and drill through metal.
If not, go to a car audio specialist. Tell them what you want. Etc…
Its not that difficult actually. All stereo parts comes with detailed instructions, also a lot of videos are available on youtube. I were able to learn from Tech Articles at carid, they recently created an awesome guide, which is really helpful!
It can be if you don't have an idea of what you have to do. I my self never had a clue but seen people install systems , so one day i got me one and hooked it up my self with out no help just by going to youtube and search videos on "how to install a subwoofer in a car". you can do it!
No, but it will take you 5x as long and you might mess something up. Find a youtube guide for your car.