The most important property is that the app cannot read/write arbitrary files/directories in your home directory [1]. All access is mediated through privilege-separated open/save dialogs or drag and drop (which creates a link to a file/directory in the app's sandbox).
I do trust Objective Development (the makers of Little Snitch), but with any application processing untrusted input, there is always the risk of compromise and its good if the compromise is limited to the sandbox of the app.
[1] Though access to certain directories also requires confirmation for non-sandboxed apps in recent macOS versions.
The most important property is that the app cannot read/write arbitrary files/directories in your home directory [1]. All access is mediated through privilege-separated open/save dialogs or drag and drop (which creates a link to a file/directory in the app's sandbox).
I do trust Objective Development (the makers of Little Snitch), but with any application processing untrusted input, there is always the risk of compromise and its good if the compromise is limited to the sandbox of the app.
[1] Though access to certain directories also requires confirmation for non-sandboxed apps in recent macOS versions.