Off the top of my head: with Forgejo, you alone have the burden of hosting your repo, which means if your repo becomes popular, you have to deal with the costs of all that traffic to it.

The nice thing about the P2P/seeding aspect of Radicle is that anyone can clone your public repo and help seed it to others.

I see that Forgejo is working on federation which should help distribute the load of hosting a repo, but that doesn't look to be completed yet