You make a very valid point. Without knowing the code it makes it a matter of proceeding with caution. I would find it very hard to trust Coinomi or any other platform right now unless they were open with their code.
p.s. Coinomi is closed-source, but who is really going to review their code?
anybody who cares a tiny bit about their security and privacy.
any of the open source and semi-popular to popular wallets that you check have gotten active contributions from at least a couple of dozen developers and have been reviewed by a lot more whether a complete review or partial.
for example i myself have one through the code of bitcoin core and electrum both of which i have used and trust.
meanwhile nobody has any clue what Coinomi does and that is a serious danger to your security and privacy both.