KYB for Fintech Startups: What are the common problems?

I'm about to move from just an idea to setting up a platform. Currently, I'm looking for a Payment Service Provider (PSP) and a broker to work with. I'm new to this industry, so I'd like to ask friends and seniors with prior experience what specific points we need to be careful about during the initial Know Your Business (KYB) process, especially regarding the required documents or even the processing time. I'm concerned that if I encounter a PSP that takes a long time to consider, it might delay our plans. Also, regarding the reliability of the PSP, are there any special criteria for consideration or any techniques for verification? I'm currently looking for a lot of information, but information on this topic is quite scattered. So, I'd like to hear direct experiences from those who have actually done it about any unexpected problems they encountered.