掲示板 Forums -

Would something like 彼はバナナが好きなのでバナナを五つ買いました be appropriate for this structure, or does it fall too far into something subjective for which から would be more appropriate?

That's a more casual sentence, so it's basically a sentence fragment (but completely natural and common in spoken Japanese).

As mentioned in the user notes, ので is just the て-form of the explanatory の + だ anyway. So both can be used to express a reason.

In your specific sentence, 難しくて sounds more natural than 難しいので to me personally, but both are gramatically correct.

I found this interesting article (in Japanese), which examines how natural ~て sounds to native speakers in different contexts: https://eprints.lib.hokudai.ac.jp/dspace/handle/2115/45574

For example, "風邪をひいて、学校を休みました。" sounds good, but "宿題をして、外出できません。" does not (it argues the antecedent should not be a highly actionable verb).