I used the free plugin too (i may be one of the first users) and then upgraded to paid. Free and paid plugins are actually same, the rules created with free plugin will even work with the paid plugin, the only difference is the links in posts.
I can’t say i loved the free links in the posts (that is why i upgraded) but i don’t think is anything wrong with it either. Most of the links i noticed in my posts were to some services or sites operated by the same company and none of them to any shady site.. There was no adult, gambling, etc.
I have had no problems after moving to paid.