It's not that they are more trustworthy - in fact, the exact opposite. Open source wallets do not require trust, since you can personally review the code and then compile it yourself. You can verify that the wallet software is going what you think it is, with no hidden surprises. Closed source wallets, on the other hand, cannot be be verified, and therefore you must place complete trust in the developers.

To expand on what bob123 has said, and since you mentioned Coinomi specifically: Coinomi was found to have a critical bug where whenever a user entered their seed phrase to restore a wallet, it was sending the words they entered unencrypted to a Google spell checking service. A user alleges that this bug resulting in him losing $65,000 worth of cryptocurrency. Now, I'm not saying that something like this couldn't happen in an open source wallet, but with many sets of independent eyes looking at the code, then even if you don't look at the code yourself it is still far better than only two or three developers reviewing the code.