Some things I found were;
Both had a mix of large (household name) and small banks offering Banking as a service (e.g. access to payment infrastructure, or specific products). Large banks worked with the fintechs for diversification and for future growth, small banks for the above plus added revenue. Banks of all sizes were always open for meetings, discussions, it wasn't restricted to small ones.
Some banks didn't understand how their own legacy systems worked at the edges, and relied on us to reverse engineer processes. By edges I mean as we were often pushing the boundaries and limits of these technologies and integrations.
All banks generally had good support, a dedicated person for day to day assistance and who could get specialists or senior people if required. Especially if there was a formal integration happening we had access to bank engineers etc. Support degraded where the day to day contact either didn't understand our business, or if they had far too many clients to look after. It also became trickier the further away the banks were geographically e.g. the other side of the world. And so face to face visits were very important.
The key bottleneck is always compliance, risk, AML, due diligence, and associated reviews. Fintechs are generally considered high risk by bank risk teams/policy and often the regulator, so there is high scrutiny. This is often a good thing however, as the fintech has to have great processes in place to meet these expectations.