It is a buggy wallet, continuing to use it means losing those currencies as there are a lot of historical incidents^[1] that have proven that they are not interested in developing the code^[2] and thus they expose customer money to loss without compensation.
It is best to avoid them and use open source wallets, no one will force you to use them.
In addition, you cannot prove that they cannot access your money. You need to trust them.


[1] @johoe return 870 BTC to the owners (Blockchain.info)
[2] Blockchain,info.  more than 12 months and still no segwit support