Quick Answer
Yuuna is best treated as a context hero, not a guaranteed universal carry. The material supports Shadow as a self-scaling Crit route, Collector as an economy/snowball route when acquired early, and Nightshade as a Poison burst or late-pivot route. Patch 0.5.3 improved Yuuna and Assassin modifier context, so old opinions need caution.
How Yuuna Works
Yuuna's core identity is Shuriken Shadow Strike. The material connects her to Assassin, Crit, Poison, Shards, Stealth, and boss pressure. She can help against specific boss needs because burst and kill pressure matter in several collected sources.
Yuuna's biggest weakness is timing. If her economy route starts late or her kill chain never begins, the build can stall. If she is used only as an early burst unit, she may later be sold or pivoted away depending on the run.
Specializations
The Shadow is the best-defined self-scaling route. It fits Crit and Assassin-style plans. The Collector is the best economy/snowball route when Yuuna is acquired early enough for the Shard or kill engine to matter.
The Nightshade is the best-defined Poison burst and late-pivot route. Community material connects it to high-difficulty and Red Rift discussion, but it also carries patch-age caveats. Use it when Poison support and fight timing make sense.
Stats and Teams
Yuuna wants Crit, Attack, Magic, Mana or Mana Regen, and enough defense or survival to keep contributing. Poison builds need Poison support. Crit builds need repeated kill or hit opportunities.
Team ideas include Nightshade plus Poison frontline/support, Yuuna + Nyx Assassin support, and Collector kill-economy shells. The safest answer is to pick Yuuna when the run can support her specialization quickly.