Unicode is complicated because the languages it needs to handle are, alas, complicated. UTF-8 is super simple. It's a variable-length encoding for 21-bit unsigned integers. Wikipedia gives a handy table showing how it works:
I am sure there are many aspects I am missing about UTF-8, but it is all reasonable in its design and implementation.
For reference, I was converting between code points and actual bytes, and also implemented strlen and strcmp (which for the latter the standard library apparently handles fine).
Having a catalog of standard numbers-to-glyphs (or symbols or whatever, little pictures humans use to communicate with) is awesome and useful (and all ASCII ever was) but trying to digitalize all of human language is much much more challenging.
Sometimes (and this can even be an admirable choice) in some specialist applications it's acceptable to decide you won't embrace the complexity of human language. But in a lot of places where that's fine we already did this with the decimal digits such as in telephone numbers, or UPC/EAN product codes, so we don't need ASCII.
In most other places insisting upon ASCII is just an annoying limitation, it's annoying not being able to write your sister's name in the name of the JPEG file, regardless of whether her name is 林鳳嬌 or Jenny Smith, and it jumps out at you if the product you're using is OK with Jenny Smith but not 林鳳嬌.
You might think well, OK, but there weren't problems in ASCII. The complexity is Unicode's fault. Think about Sarah O'Connor? That apostrophe will often break people's software without any help from Unicode.
It really is just plain text. Human writing systems were always this hard, and "for many years" what you had were separate independent understandings of what "plain text" means in different environments, which makes interoperability impossible. Unicode is mostly about having only one "plain text" rather than dozens.
It is not mandatory that your 80x25 terminal learn how to display Linear B, you can't read Linear B and you probably have no desire to learn how and no interest in any text written in it. But Unicode means your computer agrees with everybody else's computer that it's Linear B, and not a bunch of symbols for drawing Space Invaders, or the manufacturer's logo, if you fix a typo in a document I wrote that has some Linear B in it, your computer doesn't replace the Linear B with question marks, or erase the document, since it knows what that is even if you can't read it and it doesn't know how to display it.
One way I put it is, imagine if one of the first-class outputs of the Unicode Consortium was standard libraries for different human languages for different computer languages.
Except that's not true. The ASCII control codes were never glyphs, but were used to control the hardware.
You're quite right that, uh, meta-linguistic symbols are also in there and that does kind of complicate my argument.