The mobile application probably also implies some kind of server-side - in fact, the server will be connected to the Callback API and will receive notifications from the VC about events in the group. If the server is not under your control, but a freelancer, then you do not control who has access to the data. Possible leakage of events in the group.
In particular, competitors may be interested in your fresh audience - those who have just joined your group, which means a live account that shows interest in your subject matter. The “hijacking” of fresh entrants is a fairly common mechanic. Many means you can find those who have recently entered into any public. Next, they write drugs with an offer at a discount to buy the same, or to join another public, or sprinkle the original group and their business with negative.
Callback API is the fastest way to intercept a new member, because the event is transmitted to the server instantly as soon as a person enters the group. But since This API is available only to the owners of the group, it is usually difficult for competitors to reach it. In the case of an incomprehensible freelancer and his server, this task is facilitated (theoretically).
Probably, the application is not limited to reading the data. Other API methods will also be involved - sending messages to users, moderating comments. In this area, there may also be shoals - if, for example, all those who turned to the community with the message will get an answer with bad content)
Finally, the worst possible cant that the application will remove all subscribers will post illegal content, which will lead to permanent blocking of the community. Theoretically, this is also possible.
This is all theory. In my opinion, the most real threat, as Pavel Mayorov wrote, is a freelancer who got the money and disappeared, so without really doing anything.