The main drawback seems to be that some networks still refuse to peer with them so not everything is reachable.

A bigger drawback is that you end up in a bad neighborhood. 20 years ago, most traffic from tunnelbroker users were people excited about ipv6 with isps that didn't care about it. In 2026, nobody is excited about ipv6 anymore and tunnelbroker traffic is mostly abuse or trying to circumvent georestrictions... Expect to fill out so many captchas if you set it up.