Just remember that most of the benefit is in the stat boost. Getting a 5% base stat increase is nothing to sneeze at… Adds to the hero survival & tile damage etc…
As an example, see the difference in stats for a couple heroes:
Then if we add in a couple of the new heroes to the comparison (Finley, BK, White Rabbit, Alice & Seshat & Kingston) you can see that, with the costume & bonus, the old S1 heroes now can compete:
In terms of the 3* costumes I think they are all great. All offer either a direct improvement over the original skill OR provide a gap that was missing in the 3* hero rank specials:
Hawkmoon = attack buff & HoT.
Brianne = mini Athena
Gunnar = other half of wilbur
Tyrum = second cleanser (only other one was Gil-Ra)
In the 4* ranks, I think most are good. They offer something different.
Rigard = only purple attack buffer
Melendor = only Green defence buffer
Sonya = one of 2 fast cleansers now (Zimkitha being the other one)
BT = Monk healer & also more healing (trade off in less attack buff)
The only one who doesn’t make as much sense is Li Xiu (as you pointed out). Idk much about skittles so we don’t talk about her 
she doesn’t get much use or love anymore from anyone haha
LINK - spreadsheet used to generate the images: