肏 has always stuck me as a bit disturbing. It means fuck. This is a compound ideograph. The top part 入 means to enter. The bottom part 肉 means flesh. It creates a disturbing image.
I think a lot of people find the character a bit off putting as well. It isn’t used much. If you don’t want to use this character because you think it is too graphic, you can use a phonetic loan character 操 instead. The component on the left is 手 which means hand. The component on the right is 喿 which means birds chirping. There are also other characters that people use instead. But I don’t think those usages are quite the same.
凸 and 凹 strike me as ugly characters. Purely on the grounds of their form. I particularly dislike 凸. It is used in some phrases. And the form of the character sort of clashes with the mental image the phrase is supposed to create.
Anyways, there are very few characters that I dislike on grounds of form alone. I suspect a calligrapher has much stronger opinions on such things.