There is no backdoor. There is no reason to believe that.
Ledger firmware is not open source for technical reasons, not because they are scammers.
All ledger nano applications are open source.
The firmware is not open source for mainly legal reasons. Due to them using the secured element and having to sign a NDA (apparently), they arent allow to share code tied to the secure element. Rather if this is true or not is yet to be seen, but could still have the firmware open abit more without much exposure. Also, I never implied that they (ledger or any company) are scammers, however I wont rule out a "backdoor" either since. Not saying that ledger implemented directly either and keep in mind im also referring to zero day exploits that cannot be easily discovered like you could be able to find out with trezor (or other open source hw wallets) through auditing the code and have a understanding of how the hardware works with little to know reverse engineering.
Which goes back to what I said. How far down the rabbit hole do you want to go?
Let's say I make a secure element, the "Dave Chip". Knowing how it works only gets you so far. Unless you can really really really understand the microcode & design of the chip knowing how it talks to the application are only going to get you so far. Look at Meltdown and Spectre as prime examples. Yes, CPUs are vastly more complicated then a security chip, but there are also a lot less eyes on it. Same thing with the M4 that is in the ColdCard (I don't know what CPU the others use) if there is something lurking in there we may never find out.
It's all about reasonable security. I would think the bigger security issue would be there are probably still more people using the same 4 digit pin for their phone VM and their ATM card and their ColdCard then there are going to be vulnerabilities in all the hardware wallets combined. But, we can't stop that.
-Dave