Short version, Condition Firebrand and even Guardian in general isn't in a very good spot at the moment in PvE. The only reason people call it 'strong', is because the class is very versatile, which's totally different from being broken.
- Radiance DH is okay, but it's not a meta DPS in any gamemode currently, it's actually one of the worst classes in raids and sub-meta in fractals
- Virtues DH is fine but depends a lot of the group and boss, making it very hard to play correctly in pug
- Condition FB is fine, its bench is only slightly below top-tier condition classes for raids. For fractals, it's one of the best condition classes due to the fast ramp-up from burning. Mainly used in 100 CM for its utilities, does okay on the rest but not better than meta classes correctly played.
- Quick FB is currently inferior to StM chronos in many ways in raids, it's only used in fractals since chrono won't benefit from good slow uptimes
- Heal FB is inferior to Heal Renegade in almost any situation in both Raids and Fractals, given that Quick FB does more DPS than normal Alac while being given the possibility to retain all of its utilities. Better going with Heal Ren in almost any situation.
TL;DR : The first strenght of Guardian is to be very versatile, i.e it does everything okay. The second one is that the rotations are not really complicated to execute, although there are many variations and reaching benchmark is still very hard in my opinion. Qfb/DH with FMW is still meta in high-end fractals, and Virtues DH for some raid records, but that's about it. There's no way Condition FB can be considered as OP.