What's nice about Sap Sipper is that it gives Goodra immunity to Spore, Leech Seed, and the powder series of moves. I'm not really sold on the attack boost portion of it, since it gets a really nice special move-pool and suffers from four move slot syndrome. Adding physical attacks to that makes FMSS much more prominent.
I know a lot of people have recommended Dragon Tail sets, but you have to get any Faries out of the way otherwise they come in for free and threaten you out. However, it is important to note that most, if not all offensive Fairy types in OU are naturally slower than Goodra --- Mawile, Sylveon, Klefki (Prankster gives priority to non-offensive moves), Azumarill. The only exception is Togekiss, but unless you have something amazing like a boosted Sludge Bomb, you're not hitting it very hard as it has 115 base SDef, and most Togekiss are bulky nowadays. The main problem with Goodra is faster fellow dragon types, like Garchomp, Salamence, Haxorus, etc. Sure, you could room Gooey to slow them down, but I don't think the average player would let you revenge kill their slowed dragon so easily.
So, yes, I agree Goodra's ability is highly situation and dependent on team composition --- Hydration if you have enough rain support, Gooey if your team can take advantage of the slow, and Sap Sipper if you need grass immunity (which, honestly, seems the most practical to me as Goodra's typing is more beneficial than most common grass types out there).