Ah, that reminds me of the good old Attila client states. For all who haven’t played the game, in Total War: Attila you can create client states. However, each times you get attacked, you must ask them to help in the war, and should they refuse they will automatically be at war with you as well.
In practice, that means that each client state is basically a ticking time bomb: because their friend Barbaricus Maximus Chadus has decided to launch a suicide war against you, the client states often just go “yep, I wanna get annihilated too” and refuse to defend with you (and thus go to war with you).
Long story short - foederati states are trash. Never trust anyone, and remember that Roman glory is forged in blood!
Romans gave subordinate territories tribute? Seems counterproductive.
Very common for Roman client states, actually! While by the Late Empire, when it was most common, it was… probably counterproductive, in earlier periods it was actually very effective. Roman ‘subsidies’ to their client states reinforced the power of pro-Roman elites and improved the capabilities of the clients to repel their neighbors - which was usually their primary purpose, as buffer states.