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I would not ever use existing source codes.
Open source is tempting easy way to build something fast. However I always consider simple rule of thumb "If something is literally money-printing press, why it is free?" Even though I might not know what is wrong with the source, I don't want to waste time and money to find out. It can be anything from inability to handle loads to poor algorithmic or backdoors injected into it. I simply don't want to find out.
Thinking about what you just say here: without open source the bitcoin ecosystem wouldn’t even exist.
„Why is it free“ - because it can!
The egocentric, capitalistic, „free markets“ economic theories have all a poor and very limited understanding of (intrinsic or altruistic) reality. Open source brought so much to this world, that you would think, without it we would still live in caves.
The second part is way better: when handling money, you need to look into the code you‘ll be using. Rigorous testing is required, and a sound understanding, on how you want to secure your users money.
Putting yourself in a position of a customer, who gives money to an exchange, you want to be assured, that you deal with experts, and not newbies.
So open source can be a way to go, but must not. And it is not an assurance of well functioning code per se. But: anyone can help to make it - bitcoin is dealing with values in the billion Euros, and has proofed to be very, very stable against all sorts of attacks, even those trying to steal the name.