Are there any downsides to using black water-soluble pencils as regular pencils?
For the biggest part the answer to the related question is still valid. Water soluble graphite pencils are intended to be used both as pencils and as paint, and they will hold up just as well.
The appearance on paper between the two types of pencils is similar.
The only downside I can think of is that accidental smears might be harder or impossible to erase - when water is used in combination with water soluble graphite, the graphite cannot be erased anymore. Wiping a finger, wrist, or hand, especially when it's a little sweaty, can ruin a drawing more severely than when you're working with regular pencils.