The additional CIs coverage is subjected to the company's definition as they are not the 37 CI defined by LIA! It's something like "nice to have" but not "must have" because these "extra" CIs are really very very rare in nature and when it comes to claim, you are not sure how the companies define as severe or not.
Ease of claim I would say it is subjective. Some people experience bad experience here and there from all 3 companies

So far I would rate Aviva > Manulife in terms of claim processing based on experience but both companies are still relatively popular due to its competitive premiums. I do not have experience with Prudential as I do not represent them.
For insurance product, it is covered under the policy owners' protection scheme!
Refer to this for more information
https://www.sdic.org.sg/public/pp_overview