Overall I think its pretty safe but keep in mind a paper wallet is of course way more secure, with a mobile its maybe pretty secure but it still can be hacked.
True, any Sophisticated equipment that we use there is definitely a gap to hackers, as well as for the wallet in a mobile phone. There must be a gap that could be searched by the hacker, I hope if you did do a cell phone inside the BTC storage, do not use the mobile phone to open a site that is not yet known.