It's probably not as good as Leyline of the Void (although even that's up for debate, I've never attacked for nine with a Leyline of the Void), but it also doesn't take up space in your sideboard
Not being affected by special conditions just isn't enough to prop it up
With a PSA 9 grade, its condition is excellent for a card from the original Pokmon Game set
Tomik, Wielder of Law - Tomik has a similar issue to Mangara
If an effect like Hardened Scales is an auto-include in these decks, a full-on counter-doubling effect is going to be a rather exciting prospect
Whether or not you should sell a golden chocobo is a tough answer, but if you must have an answer right away, I'd find the card grading service I trust the most, get the chocobo in a slab and hold onto it