I’d say сложноватый вопрос

To me, сложный is outright difficult, while a toughie is hard, but only relatively so (hence the diminutive ending).

hi Uly, then I think "непростой вопрос" is better

You’d know better than me. I just think that the diminutive injects a slight humor into this, as if jovially preparing the listener in a mildly teasing way for a difficult task, or playing down one’s own trepidation at the prospect of taking on the task in question. That’s why I figured сложноватый would be more to the point. But again, as a rule, I don’t argue with natives 👍🏼

Difficult to explain this one, it's more of my feeling that can be subjective. The objective fact is that "сложноватый вопрос" appears neither in Google books, nor in НКРЯ (https://ruscorpora.ru/).
The scenario when I can imagine it in use: two professors discuss exam questions, and one say "это сложноватый вопрос для наших студентов, лучше его заменить на что-то попроще"

Again, my Russian isn’t good enough to see the difference. However, there are a lot of instances of it in a regular google search: https://www.google.com/search?client=firefox-b-1-d&q=%22%D1%81%D0%BB%D0%BE%D0%B6%D0%BD%D0%BE%D0%B2%D0%B0%D1%82%D1%8B%D0%B9+%D0%B2%D0%BE%D0%BF%D1%80%D0%BE%D1%81%22