Most probably this is a duplicate but I looked for some guidance on this in both help and meta and I was not able to find.

When someone is asking a question on which the answer is not directly clear, but an certain answer is likely to be true, should it first be certified by comments or is it a answer?

On a lot of questions there are many possible answers or suggestions. Since the comment area is quite small and with only minimal format possibilities I tend to prefer to give my suggestion as an answer and handle questions on the answer in this comment area.
But is this right, and where to draw the line? I see a lot of high reps doing these suggestions in the comments so I am a bit puzzled on this.

For example this link. Where should the suggestions be?