Definitely the worst advice I've been given was advice relating to how I depict gender. In my early online work, my female characters were frequently mistaken for male, because in the late 90s and early 00s, bishounen characters and ecchi were all the rage and barely anyone, even in LGBTQ+ circles was familiar with terms like "nonbinary", so I got told over and over and over "your girls look like boys!", "you need to make your girls more feminine!" "They need more prominent boobs and eyelashes". People were VERY concerned about this, as though a girl not being very very obviously a girl with exaggerated feminine characteristics was a horrible thing that needed resolving, even in a story where the character's sex or gender were completely irrelevant. My work from the mid 00s looks really awkward because of this attempt to make characters more superficially feminine, and I feel like it held back my anatomical drawing because in reality, male and female bodies and faces actually aren't all that different, and treating them like it's two totally different approaches to drawing anatomy is silly. It's much better to learn general anatomy more like it's a sliding scale like "okay, people with a more female pelvis it tends cause trousers to crease like this... Broader shoulders work like this... More muscle tone in this area will look like this..." etc.
In other words, learn what physical characteristics will make a character appear more masculine, and what will make them look more feminine and to have the freedom to mix and match and play around and to not have to worry about people getting on your case because they're angry about not being able to neatly assign gender to characters (and they're scared that maybe they're a bit bi, I dunno? 
).
Best Advice: "Do what works for you." Basically, early on with manga stuff especially, there was this common idea that to draw "proper manga" you had to use a dip pen and a pot of ink on a piece of bristol board on a drawing board, real screentones, draw right to left and draw in a style that was as indistinguishable from Japanese creators as possible. But some of the more experienced artists on the UK scene advised a more pragmatic approach. They'd give advice like "no, draw your comic left to right if you're making it in English; it'll read better", "Use the pens that you can afford and have space for and give you the best result", "Of course you can mix manga influences with other comics influences!", "Yes, digital tones are fine, just make sure you're working with the right DPI for your print size!"
In the end, it's all about what gives you the best pipeline for keeping making pages that look decent, are easy to read and express the feeling or action you want to express, and if your way of doing that is a bit different, that's okay!
