I know someone who works for a major insurance company in the US, in the appeals department. I don’t know the intimate details of everything about their job, but one thing I always remembered is if you have an expedited appeal (for a medical emergency), and they’re requesting documents that you don’t send within the 72 hour deadline an expedited appeal has, the denial gets an automatic administrative overturn - the appeal is approved. Presumably for liability reasons, combined with that the company gets fined if an expedited appeal takes over 72 hours to process. Potentially a usable strategy if you have an expedited appeal that’s weak enough to have a chance of the denial being upheld in the end, and they’re also requesting documents. Probably a pretty rare combination of factors though.

But I’ve also been told by them that most denials end up being overturned on appeal in the end. And that they see people appealing denials on a daily basis, where they were denied the absolute stupidest reasons - people who obviously should never have been denied in the first place. Hard to say if this means most denials are bullshit, or if it just means people only bother trying to appeal the most obviously bullshit ones.