Having implemented AES and a few other algorithms in "pure formula" Excel, I fully support seeing more things implemented in Excel!
One of the biggest things to wrap your head around is that cells in Excel are write-once, read many. This forces you to unroll all of your loops, and you generally end up with a giant state machine, with each state occupying a row and depending on the state of the previous row.
Yep. Additionally, there are no user-defined functions, and the limit on the length of the "code" that makes up a cell is low enough that you occasionally bump into it when trying to "program" in Excel.
Excel also has a tendency to get crashy when you have a very complex cell dependency tree...
One of the biggest things to wrap your head around is that cells in Excel are write-once, read many. This forces you to unroll all of your loops, and you generally end up with a giant state machine, with each state occupying a row and depending on the state of the previous row.