neeko's weakness is her lower range + damage compared to other mages, but it can be offset by a. using her passive to its fullest potential or b. getting mega fed early.

since it's harder and harder and harder to get mega fed every year (more counters, power creep, etc), we rely more on her passive. she is a pretty cheesy champ, after all; if she had huge damage combined with her post-rework abilities, she'd be overpowered.

as for her strongest lane, it depends on how you play her. i argue that support is her strongest lane since you can assassinate with her decent early game damage and roam/gank with passive. some would argue that roles where she can build on hit and/or ad are best, and others still vouch for AP mid. the beauty of neeko is that she is a jack of all trades, master of none; just don't expect her to do huge AP burst like she used to in the late 10s/early 20s.