Coffezilla talks about the evolve scandal below. And mentions Mercury walked away the same day that shortfalls were being discussed between Synapse and evolve

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P.S Personally I would keep no money in any of these US fintechs. Even Evolve bank came off sounding shady as hell

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Coffeezilla video is unfortunately a very inaccurate rendition of events.
Simply Synapse moved funds between client accounts at Evolve and their own licensed brokerage. When you are a client of a broker, you are not insured by the FDIC (obviously). The problem was that Synapse terms allowed them to do, but they misinformed clients by not including these movements in the statements and issuing the statements displaying only Evolve details. It invested the funds through its brokerage (undescribed how this was done, but with the unknowing consent of clients). There are more in-depth facts to this case but frankly I don't feel like rambling on for paragraphs about the exact scenario

Anyway, the point to consider is that Evolve has the reserve funds for the clients, only it does not know who it owes – and the other banks are saying it owes more than it does because Synapse supplied these incorrect statements that did not account for it sending funds to the pooled accounts of its brokerage. Evolve could have made mistakes in transferring Mercury at Synapse to Mercury's own accounts, but these are solely accusations, and I find it unlikely – because Synapse itself stated that there were separate custody accounts for Mercury funds (when Mercury went through them) due to the large sums being held – if Synapse was placing other FinTech's money in the Mercury safeguarding, this is again their fault.
Personally, I know some happy BaaS clients of Evolve and the bank seems decent except for issues with SWIFT clearing. High reserve account requirements, though.
I think US FinTechs are not risky, but you have to see where they are holding funds. For example, some hold everything in T-Bills, some are using Pershing custody (very reliable – BNY Mellon), some are holding it as just general USD with the backing bank; in the worst case, they are doing what Synapse does, sweeping deposits to their own brokerage.