And yes, we serve the 30 banks that constitute 60% of banking volume in the US, and at least 60% of banking customers (although we suspect more, since many people who have a smaller account have a top 30 account too). We're increasing that number, but you can turn our system on so that when we don't integrate, you can still send account number and we do ACH the traditional way.
As for you not wanting to login to your online banking - you may be right! You may never do it. However, I've heard that before from a lot of people who turn right around and use Knox 30 seconds later and then become customers - so I hope you're in that group! There are some people who will be cautious, but it's actually smaller % of people than I initially thought it would be.
I would recommend configuring support for "Forward Secrecy" on both sites and enabling TLS 1.2 on thesimpledifference.com. Current score on both domains is "C".
Why? iFrame SSL certificates aren't visible, and no, your VeriSign trusted doesn't count. This means users will find it offputting as you note.
Why else? Someone could hijack the HTTP page and point the iframe location somewhere else where they then intercept bank details.
Next knox only seems to support a limited number of banks, unlike ACH which I'm assuming supports alll banks.
Finally 'log into their online banking' worries me. I'd never hand over my online banking details to a third party (I suspect doing so would violate the T&Cs on my account!).