Both the Shulchan Aruch Harav and the Mishnah Brurah use similar terminology when describing the importance of the shin being "pointy" on the bottom and all three branches of the letter shin meeting at a point or "chad" at the bottom of the letter. There is a strong foundation in Halacha for this and for the bottom of the shin to be flat like a moshav (base) is considered questionable (Pri Megadim) and definitely not Kosher Lechatchillah. It is worse if the moshav is very wide, but it is still questionable if it is lechatchillah if there is a thick noticeable base rather than a chad.
Even for Sephardim, who lechatchillah make an angular base, it is still important that the base is indeed on a (significant) angle. If the base is flat, even if all three branches of the shin come out of the base connected , as in the top picture, it is problematic. It is worse in the bottom picture below where the right head/ branch comes out of the right part of the base and the middle and left branches come out from a different part of the base.
(Pictures are from sefer Ksivah Tamah)
In the Alter Rebbe's Ksav, the shin is one of the most difficult letters to write correctly. This is because the right branch of the shin curves and gets substantially thicker as it comes down, almost like an ayin. But it is important to ensure that it still meets the requirement of all three roshim meeting in a "chad" as described in S'A Harav. It is very easy to slightly err and inadvertently create a (flat) moshav. In some cases this can be problematic and may need to be fixed.
Over the years I have seen some very exaggerated cases, where the bottom of the shin looks more like Mor Uktzia ksav than Alter Rebbe ksav.
I recently saw a very beautiful Alter Rebbe mezuzah where most of the shins were done very well, such as these:
But some (few and far between) in the same mezuzah looked like this:
I stress that these are mild cases (and not the very exaggerated type I refer to above). However these still have a clear, distinguishable moshav and are flatter than ideal, and hard to really consider Chad kra'ah . IMHO if such a Mezuzah is being sold as "high end mehudar" it would be worth fixing the few shins written like this with an ever so slight adjustment to create a better chad.
Does the oilam agree?
UPDATE: Someone emailed me this picture highlighting what he felt to be the most incorrect aspect, namely the almost 90 degree angle between the right branch and the moshav.
Although I touched on this, he wanted me to reiterate that this is a common mistake and the inside should be round / curved ie one flowing aiver and not have such a sharp angle which essentially exacerbates the problem.