Hello there and welcome to the threads. The current version is actually 3.5, whereas v3.6 will be released in the upcoming months. I believe I've explained a bit about Nair in the Lucas guide, but here's a quick breakdown.
Summary
Strengths: beats out crouch cancels especially when followed up with Dtilts. Great on shield. Potential followups based on DI and the last hitbox that connects with an opponent.
Weaknesses: Multi-hit, doesn't trade well, low priority, small range, knockback only results from the latter hitboxes.
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Explanation
Nair has 2 types of hitboxes- a multi-hit aerial hitbox with knockback at the end of the move, and a grounded hitbox upon landing (if the move hasn't finished in the air that is). This makes Nair a great move for shield pressure & beating out crouch cancels.
SHFFL Nair links fluidly into magnet, grab, or Dtilt depending on the opponents DI and the hitbox that last connects. If you connect with one of the first few hitboxes before landing, the opponent will not travel very far at all, and in some cases stay in place, especially if CC'ing the move. This can lead into magnet if they attempt to retaliate afterwards but have too high % to CC the magnet, or for safe pressure on CC foes lead into Dtilt (frame 3, good hitbox, instant knockback unlike magnet).
As for shield pressure, Nair does great shield damage and can take almost a third of a shield with one full hit. The ground hitbox also adds to the shield damage and in some cases can shield stab. This will allow for some good followups if you land only the grounded hitbox.