When is the Flyweight pattern actually worth applying, and what makes it risky to reach for by default?

Flyweight is worth applying only when there is a genuine, measured memory problem coming from a very large number of objects that share substantial identical state -- the kind of situation where doing the actual per-object byte math shows a real, significant cost. Applying it without that measured justification adds real complexity for no benefit: a factory managing a shared cache, and a mandatory split between shared and per-instance state that has to be maintained correctly everywhere the object is used. It is one of the most niche patterns available, and the risk of over-applying it is genuinely higher than the risk of missing an opportunity to use it, since most object counts in ordinary applications never get large enough for the savings to matter.