It'd depend on the species, but the blue-leg hermit (Clibanarius tricolor) does perfectly fine in brackish water, and has been kept in as low as 1.008 SG.
Considering tricolor is found in estuarine environments, I'd imagine they "tolerate" brackish quite well. Though I'd admit I've been out of the brackish game for quite awhile, I haven't heard of anyone experiencing any differences between keeping them in brackish and saltwater.