Any company, from Big Banks (Wells, Citi, BofA) to Mortgage Bankers (Quicken, loanDepot, Guaranteed Rate) to micro broker shops (Barrington Financial, etc) are asked to provide the exact same level of basic documentation as everyone else does. Big Banks will have Big Guideline Overlays and can ask for considerably more documentation than other shops might, but on whole, the basic package is the same from company to company. The issue isn't with the funding lender's requesting of more information. Once the loan is approved, most closing processes are akin to landing a plane well on it's glide path. The issue with documentation and underwriting, the ask, and re-ask, and ask again for documentation lies with the originating Loan Officer, and no one else.
The best way to avoid these kinds of problems? When applying, perhaps using a call in center isn't the best way to handle complex applications. Pricing in call centers (Owning/Quicken/Fremont Bank and loanDepot) tends to be better because the staff is often new and not highly compensated. That reduced compensation goes into reducing the rate - the reason why many will apply with a low rate provider. If working with a loan officer directly rather than a high volume call center means .125 in rate, or $1,000 in fee more, it might be worth considering that route rather than chasing a very low rate but going through an immense hassle to get it.
The second best way to avoid processing issues? Ask the loan officer when they started in the business. If it's within the last 2-3 years, perhaps you might want to work with someone with a longer resume - someone who has seen more complex applications through successfully to closing than others may have. Each loan officer has an NMLS ID. You can look up their employment history here:
https://nmlsconsumeraccess.org/ PM me and I can give you mine if your curious. I've been advised not to openly post it on blogging sites.
With high volume comes longer wait times. Processors in some cases have 100-300 files PER MONTH to close. It's not fair to the customer, or to the mortgage processing staff to be in this situation, but it's where the industry finds itself today. With high volume comes errors, duplication, and other crash and burn scenarios. It's not fun to be spun around during a loan processing, but you can avoid many issues by working outside of a call center environment and with a seasoned professional should your specific employment, cash, or property situation be complex in any way.
My .02c