Mods that bypass payments (whether providing paid apps without payment to the developer or bypassing in-app purchases) are app piracy, and as such we don't discuss them here. The
forum rules make clear that providing links to such apps can lead to being banned from the forum, so it's obviously not in the spirit of the rules to provide any support for them.
Strictly speaking, any mod, even to a free app, is going to involve infringement of copyright. Even redistributing an unmodified free app without the developer's permission does that: the fact that the developer does not charge for the app does not
in itself give anyone the right to do anything other than use the app without charge (of course they may be perfectly happy for people to redistribute or even modify, but you need to check before doing so). So legally we're on shaky grounds in providing any assistance with these things unless the developer has given permission for modifications to be made (and of course we don't want to cause trouble for the forum owner).
But safety? That depends entirely on who has modified the app. Generally speaking I'd advise extreme caution: unless you are sufficiently technically competent you have no way of knowing whether the modifications include a malware downloader, a keylogger, adware, etc. It comes down to trust, and in many cases you'll be asking whether you trust someone who has already shown willingness to break laws to rip off the developer not to be ripping you off as well. Yes, some people will hack apps just to stop them tracking you or things like that, but as the person who downloads and installs it you have to decide for yourself how confident you are that there is nothing malicious in there.