I was consulting someone on IELTS writing recently, when they asked me, "I heard in one video online that it's best to stick to obvious ideas in IELTS essays. Do you agree?"
No, but maybe it's just because of my argumentative nature. (Just kidding.)
On a more serious note, there are two things to keep in mind:
1️⃣ The overwhelming majority of IELTS-related content online is made for scores 6-7 because this is what most people need. For these scores, indeed, obvious ideas are good ideas because they save time and are easy to write about. People at these levels struggle with language already, why add one more struggle?
2️⃣ At higher levels, I think counterintuitive ideas work better because they can give you more opportunities to develop them in depth. Since you expect people to disagree, you will need to explain a bit more. More explanations = more depth and more good language. And if you go for a predictable idea, sometimes all you can write is a bunch of clichés that you think are self-explanatory - and this is a surefire way to lose the proverbial depth.
I have written many a counterintuitive essay
On an even more serious note, I've written many essays in two versions: one "agree entirely" and one "disagree entirely." Because how else will you know which works better if you don't try both?