For me? I installed it and set it up according to the instructions, didn't work. Reported it, got tech support, still doesn't work. And I never got a good reason why, just that "it always works."
Most people love it, so I'm not saying its not great. Just that all I get is errors
Are the indexers (torrent sites) you are trying to add to jackett blocked by your ISP? If so, they won't work in jackett without a VPN, just like how they won't open in a browser without a VPN.