Nah dont worry about any long-term downsides to using spartan rage, its actually designed to help you out when things get tough. I've used it tons of times and the only thing you need to think about is waiting for the meter to fill back up after using it. Its pretty straightforward, just hit l3 and r3 together when youre ready to unleash it, and youll get some really nice perks like healing up while dealing damage in fury mode, or getting those satisfying counters with valor.

The cool thing is, the rage meter fills back up naturally as you fight, so you can actually use it pretty regularly without any worries. I usually save it for those moments when i need to turn a fight around or when i want to interrupt some nasty enemy attacks. Really helps when youre in a tight spot, and its actually meant to be used as part of your regular fighting style, so dont hold back thinking itll hurt you later on.