The bulk fighting forces of both the Dacians and Thracians was indeed the tribal levy, ie. freeman peasant-warriors. Much like the Germanics that way. But thanks to the local geography and other military circumstances their method of choice was skirmish-heavy light-infantry tactics, something which they were past masters in and which caused overly ambitious Greeks from the south no end of pain.

The point being, they are the backbone at least early on; but they're not a line infantry backbone, although scrappy enough that they work surprisingly well as such if necessary.