Alolan Exeggutor has a slew of very exploitable weaknesses, including a 4x to Ice Beam, which is probably the 2nd most common attacking move in the game, after EQ, and a 2x weakness to U-Turn. That, coupled with its rather mediocre bulk, and atrocious speed stat makes our eggy friend unable to have a niche in RU or higher, since its weaknesses are just that common. Except on gimmicky Trick Room teams. However, in NU it is a monster, capable of sweeping teams in a moment's notice thanks to its massive special attcking movepool and huge SpA that can be further boosted by Choice Specs. It is able to check things like Wishiwashi, Jellicent, Lanturn, Gastrodon, Runerigus, and Sandaconda, among others.
*Also Alolan Exeggutor is in NU not PUBL
Hope this helped!