I have only used the mobile version for small amounts in the past. I haven't had any problems with this. I wouldn't use it as my main repository, I have other solutions for that.
It is easy to use, supports a wide variety of cryptocurrencies and provides fast exchange between Shapeshift and Changelly and coins. If I remember correctly, there was even the option to buy crypto with credit card or buy gift cards with Coinomi.
and to summarize;
It is convenient and supports a large number of coins that most people want to have in their wallet. and it is easy to use.
but;
closed source !!!! Although GitHub has some arbitrary code, it doesn't match what they post, and the code is rarely updated! which is another red flag.
has done some shady things like sending your seed to a server in the past.