1) No easy way to try outside a framework (now there is with Parcel RSC, but eventual first-class support in Vite will likely be the real tipping point).
2) Poor developer experience in the only official integration (Next.js). Both due to build performance problems (which Turbopack helps with but it still hasn’t fully shipped), due to bad error messages (massively improved recently), and due to confusing overly aggressive caching (being reworked to much more intuitive now but the rework won’t ship for some time yet).
Time will tell but I’m optimistic that as there are more places to try them, and the integrations are higher-quality, people will see their strong sides too.