https://www.gamespot.com/articles/destiny-2-faction-engrams-only-reward-you-with-gea/1100-6453632/

We know that Destiny 2's Faction Rallies are only limited-time events, but even within the confines of that, you won't be able to secure an unlimited supply of gear from factions. Only a limited number of the rewards you receive will be of the non-cosmetic variety.