The Problem:
Both of these summons are terrible. This is owing mostly to their comically low attack bonus. For example the shadowfiend has +3 AB, which could be surpassed by a level 1 wizard with 16 str.
This is not good for a mid to late game summon, which is when you can unlock them.
Anecdote:
This was driven home hard when I tested mine. I sent one of these against a single Agarat (cr 4-5?) and the shadow fiend was unable to defeat it over the duration of the ability. This means this summon is essentially useless. As this is a one hittable, no threat, enemy for my main character.
Solutions:
Obviously these creatures need to either have a much higher AB, or to make non-stop shadow attacks.
I believe the actual shadow fiends encountered in the forests of Barovia are much, much stronger than this. So maybe we should replace the summon with that?
If the devs are open to it, I would even say to completely replace these two summons with other
shadow plane creatures. Maybe a greater shadow and a shadow demon? Those are also in the module.