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Here's how I see it:

A sweetheart as a character class is just fine because it brings positivity, however, usually the sweetheart does nothing but act cute, and as a result, becomes a one note positive character. This is fine in overly negative seasons where any cuteness and sweetness is appreciated. But in seasons with plenty of characters to root for, it's easy to not give a shit about a sweetheart.

Borneo: Kelly was someone to root for, as was all of Pagong, thus Colleen was forgetable to me.
Australia: Rodger, Tina, Colby, Michael were all good people to root for, so Elisabeth was forgettable.
Africa: The Elders were root-worthy which is already a strike against Kim, not to mention Ethan, Big Tom and Kim P., so Kim P. was forgettable.
Marquesas: Kathy was someone to really root for, as was the entire underdog alliance and Hunter, Gina, and Gabe, so Neleh was more annoying than forgettable.
Thailand: This would've been a great season for a sweetheart because of the overall negativity, but Tanya was an early boot.
Amazon: Same as Thailand, but all the young girls turned out to be bitches.
Pearl Islands: Same as above, but Michelle was an early boot.
Vanuatu: Same as above, but Dolly was an early boot.
Palau: Didn't need a sweetheart because of the underdog Ulongs and Tom/Ian.
Guatemala: Could probably have benefitted from a sweetheart, but it was a pretty good cast as is.
Exile Island: Even though I think Sally was boring as hell, I suppose she did a good job as a sweetheart. She was just majorly overshadowed by all of Casaya and Terry, hence forgettable.
Cook Islands: Didn't need a sweetheart because they had a lovable tribe of underdogs in Aitu.
Fiji: Holy shit, this is why Michelle was so great for Fiji. There was just absolutely so much negativity on Fiji that even Yau-Man couldn't balance it out. You had Lisi who was a vile bitch, Rocky who got far too much screen-time and was a complete asshole. Toneless women who were picked off left and right. The four horsemen that were despicable. Amidst all this, Michelle was the cute little girl that was needed to keep the season from going completely down the shitter. When the four horsemen booted her, that was just to make them even more hated. Plus, you can't blame her for getting screwed by the editting given that she was screwed by a twist.
China: Amanda was the sweetheart of the season, somehow, but there were people like Peih-Gee, Denise and James (ugh casual viewing audience) along with Erik and Frosti that were root-worthy.
Micronesia: Erik was the sweetheart and he was DEFINITELY needed, given just how negative the overall season seemed to be.
Gabon: Sugar was obviously the sweetheart but she was actually more multi-dimensional than that.

In conclusion: Michelle Y. and Erik R. are the only really successful sweethearts because they came from seasons that were overly negative and their positivity helped prevent that negative from overwhelming the season as a whole.