I’m not going to lie to you. I don’t really know much about Alchemy as I’ve never tried it before Apple acquired it. But everyone says how great and powerful it is, so there you go. Checking the Logic Pro X 10.2 Release Notes, there is a huge list of fixes, changes, and performance improvements to the app besides the addition of Alchemy. This list is much more than what the App Store shows. (On the other hand, the MainStage 3.2 Release Notes are similar to what you see in the App Store update.) It will be a while before everyone has a chance to try Logic Pro X 10.2 and see what they did, and if they fixed issues some people were running into consistently.
This is a good article about changes to Alchemy and Logic that may be of interest to Alchemy users.