my biggest issue with hp ground and the like is you're trying to turn clefable into something he's not -- a reliable sweeper. he's a super niche pokemon, that will shine in certain very specific scenarios. and it's best to treat him that way. you have to play within in his element and simply admit that he will never be flexible like a snorlax, zapdos, or even gengar. he's going to be flat out useless at times. hp ground is a too much jack-of-all-trades cover to try and fix weaknesses instead of playing to his strengths.
in a realistic scenario, i don't see clefable getting off more than 1-2 drums per fight. when you drum with fable, you drum to win. you don't drum to weaken something. even with moonlight, i think clefable has way less staying power than a tentacruel [primarily due to typing, but also stats]. you simply cannot drum if you take any sort of damage vs a faster pokemon (clefable is slower than ttar for comparison). using hp ground puts you 100% reliant on your drums. having a fire blast, ice beam, whatever lets you attack in the event you've taken some damage.
playing for luck is never a bad thing if you need to.
in a realistic scenario, i don't see clefable getting off more than 1-2 drums per fight. when you drum with fable, you drum to win. you don't drum to weaken something. even with moonlight, i think clefable has way less staying power than a tentacruel [primarily due to typing, but also stats]. you simply cannot drum if you take any sort of damage vs a faster pokemon (clefable is slower than ttar for comparison). using hp ground puts you 100% reliant on your drums. having a fire blast, ice beam, whatever lets you attack in the event you've taken some damage.
playing for luck is never a bad thing if you need to.