I write clear across the spectrum.
Take Frankenstein's Family -- Victor's gender dysphoria is very well concealed, Adam's issues are physical but fairly subtle, and Igor's can't be hidden even under a heavy coat. A whole bunch of people have what in modern terms would be variously classed as PTSD and/or depression mostly due to surviving horrible traumas.
An Army of One is prevailingly, though not exclusively, neurovariant of various flavors. Most of these characters, you wouldn't spot anything different -- until they start squealing, flapping their hands, and showing through behavior that their brains work differently than neurotypical people. But they're the dominant culture now, so in in the Lacuna it's, "How are we supposed to know you're upset if you don't flap your hands?" Astin and V have added gender dysphoria. (Just being trans, in my mind, is not a problem; but when society mashes you for it, that can cause problems.) Spalling has PTSD out the wazoo because reasons. About a million of them. 0_o But he's not the only one, just one of the more vivid examples.
In Path of the Paladins, the disabilities are magical as well as mental. Figure most or all of Gailah's surviving paladins have some combination of PTSD and depression, which since the world is a mess, so do lots of other people. But they also lost most of their magic. They deal with it variably -- Shahana just won't quit, whereas Johan has been a nearly-suicidal wreck. Ari has been working through rape trauma.
Monster House has a blind character. Blindness, like deafness, is one of those disabilities that doesn't necessarily show at first glance but soon comes clear through behavior. Some people can pass as ablebodied; others can't. It makes for an odd half-and-half position in the world, which shows in a few places within this series. That she compensates for the blindness with a magical artifact, a white cane, and a seeing-eye gremlin does not change the underlying disability, just reduces the amount of limitation. She still moves through the world differently than a sighted person does.
With Tripping into the Future, the viewpoint character is sooooo depressed. I'm not sure if that really counts as hidden or not since there's nobody else around.
One God's Story of Mid-Life Crisis is all about substance abuse. It deals with self-control, shame, reliability, addiction, a whole mess of related issues. This is a disability that some people can hide when they're sober, and others can't hide at all. Trobby, he doesn't even try. He's a drunk and a priest of the God of Drunks, and he's half-proud, half-ashamed of it. He and Shaeth are good for each other, though. I guess in the end, this series is really about acceptance.
Polychrome Heroics has a whole slew of things. Several characters have partially or wholly lost their superpowers. Cassandra not only lost hers but has been living with chronic pain for months; getting better now, but it left her with a lot of crap to overcome. Damask survived a psychic assault to become a multiple system -- functional enough that they've managed to fake being Maisie for months, despite a lot of trauma. Mallory tried to commit suicide and wound up with superpowers. Green Man is mentally damaged from how he got his powers, and he really can't hide it for long. Haboob is downright psychotic due to a combination of physical and mental trauma. There are several characters who are blind or deaf, and it varies whether that's their Poor quality or not; some of them have a different thing that messes up their life more. The Rescuer is a survivor of child sexual abuse, and he's not the only one.
In Walking the Beat, Kelly is Deaf and Dale uses a cane to compensate for her limp. Both of them skirt the boundary between visible and invisible.
I have to mention "The Lotus Warriors," a stand-alone rather than a series, featuring chronic fatigue. In my writing, anyone can be a hero.
Really, I could not count how many characters I have written with PTSD or PDSD ranging from mild to crippling. Dozens. Possibly hundreds by now. Sometimes it is subtle, only hinted in symptoms as the character momentarily loses enough grip to give it away; other times it is open. Hypervigilance, insomnia, nightmares, flashbacks, etc. Much the same is true of depression, anxiety, and the other mental issues that tend to arise after horrible experiences. It doesn't affect everyone, but it's always going to hit some. And since a lot of my characters get into trouble, I'm honest about showing the aftermath.
Have I missed any invisible disabilities that you think might be interesting to explore?