I don't know enough about SSIS to know about the Pre and Post methods, and I'm at a loss as to the scoop with the post one not running, but I would suspect that there may be things you just can't do in post, possibly due to scope problems, but I'd be taking a giant SWAG with that.

I'm not sure why you need to have the package count the transforms though, when it seems that just having a derived column detect the conditions necessary to cause one, and then outputting a column whose value is Y or N for transformed or not for each one you want to count, might be easier to document, and very easy to query later, without having to depend on the package. You'll need to keep the data in the same table, or you can multi-cast it to a separate table with the same primary key. It also gives you a permanent record of the transforms in your database at the record level, which you just never know when that might turn out to be awfully darn handy. However, there may be other considerations that come into play that you have yet to detail. I'm also wondering what kind of transform you'd want or need a script task for. In other words, there may be other ways to "skin the cat", so to speak.

Other considerations could include things like data volume, performance, or even the very nature of the data. Wish I could speak to the "when" part on execution. I'd like to know that myself...