I recently switched from projectile to project and I noticed essentially zero changes. If you're not doing anything particularly custom you might not even notice.
https://www.gnu.org/software/emacs/manual/html_node/emacs/Pr...
As far as I'm concerned, project.el lacks the following features OOTB:
1. Something like the .projectile file to mark a folder as a project (without VCS).
2. Support for ag, rg and vterm.
3. Register new project types (to customize configure commands, compile commands, etc.)
4. Regenerate tags.
5. The ability to find the "other file" (x.h <-> x.c) across the project.
And this list is incomplete! So if you rely on any of the above, I suggest not migrating to a project.el-only setup.