My TV is insulting like that. It technically has an EQ, but it makes no perceivable difference no matter what I do in it.
What the hell!
But assuming it worked, wouldn't doing that strictly with sound frequencies cause issues? Like, okay, most voices are louder because I boosted their frequency, but now that one dude with a super low voice is quieter, plus any music in the show is distorted. Or something like that.
Not necessarily. Regardless of vocal range, around 400hz-2000hz makes up the body of what you hear in human speech, or the notes for instryments carrying a melody. Below that, say, 160-315hz is going to be the "warmth" and "fullness" of the sound, while 2.5khz-8khz is going to be the enunciation and clarity (think ch-sounds, ess-es, tee-s, etc).
Sure, if you start really going hard on an EQ, you could absolutely throw everything out of balance --- if you cut out 12db at 250hz, all the warmth will be gone and everything will sound thin. If you scoop a bunch of 400hz-1.6khz, it will sound like a walkie-talkie, and if you make a large boost around 3khz-8khz, then everything will probably sound harsh and scratchy.
This is where, the listening environment becomes important to consider. Do you live near a busy highway or do you have a loud air conditioner? You don't need to answer these questions in public, but those kinds of ambient sounds can compete with the enunciation frequencies, or add to the buildup of "mud" in the lower part of the spectrum.
The size, shape, material properties etc. of your room and furniture also play a role here. For example, a bunch of bare walls and hard surfaces will cause a lot of the high frequencies to bounce around, potentially causing a buildup of harshness. This is why recording studios and your high school band hall probably have those oddly-shaped, cloth-covered wall "decorations" that serve to neutralize the cavernous sound you'd get in a large, bare room.
Overall, compensating for the environment is where you should probably aim your EQ. That is, even if source material varies wildly, it's probably best to try to EQ to the room you're in rather than each, individual program.
The way to do it is to find a song you know by heart, that you know how it sounds in the best way possible (there are a few that, to me, sound great in my car and on my favorite pair of headphones, so I use those), and play that through your TV. Then, fiddle with the EQ until it's as close to the ideal sound in your head as you can get it.