Is it really safe to save Bitcoin and Altcoin in Coinomi? Why is the import private key in Coinomi charged?

Dear Steem Lover..

I have successfully tested the import paper wallet dogecoin (privkey) into the Coinomi wallet at a cost of 1 DOGE. However, when trying to import the privkey to my.dogechain.info it did not work.

Then I tried the opposite way ..
I tried to create a new DOGE paper wallet and I loaded 100 Dogecoin to the new address.

Then, I tried importing the privkey into my.dogechain.info wallet and it worked, no cost at all. Then I tried to enter the privkey to Coinomi, can it?
It turns out it still works, at the same cost as 1 DOGE.

The important differences here are:

1. Why import privkey to Coinomi there is a cost, whereas privkey is the core part of altcoin itself.
2. Why are privates who have signed into the Coinomi wallet not moved to another wallet, whereas in their promotion it says they are not holding user funds.

So .. what happens if the Coinomi wallet is inaccessible ?? Of course Bitcoin and Altcoin funds users will automatically disappear.

Please give opinion....