I just finished (except for weathering) Walthers Prarie Star Milling plant. I purchased it thinking it was a modular structure but found out that it is actually a hybrid, part kit, part modular. The four lower level walls and two of the upper walls are kit parts that mate with the modular components. This has several advantages in that you could easily add addition floors, or substitute different wall sections or roofs to customize the kit as you like. Also, if you mess up one of the modular sections, it is easily replaceable. Also, there were more components (pillasters, cornices, windows, etc.) included than were actually needed to complete the structure so that was a bonus as well.
I have 3 other structures on my layout that were purely modulars. Nothing fancy, just very boxy buildings. The problem I have is that you are left with a very visible seam between the upper and lower wall sections no matter how tightly you get the them to fit. I wonder if anyone has a solution for this. It seems to me that it would be difficult to fill this with plastic putty without obscuring the brick detail around the seam.