Unicorn Jabu is a straightforward cleanup DPS built around finishing weakened enemies. At max skill, his passive adds 30% extra damage against targets below 40% HP, so all parts of his kit become much more threatening once the enemy team has already taken some chip damage. Unicorn Gallop gives him an AoE opener with a reset mechanic on kills, while Ancient Consonance offers a stronger single-target sequence for closing out one unit. The design is simple, but it is easy to understand and works well for early progression.
His problem is ceiling. Jabu does not bring hard control, protection, or scaling that keeps him relevant in serious PvP, so he is mostly a beginner damage dealer and a collection unit later on. He performs best when paired with allies that soften the enemy team first, allowing him to chain kills and convert his reset into momentum. For new accounts he is serviceable and cheap to use, but long term he is quickly replaced by stronger physical attackers.