E.g. please give me guidance on how to better structure my database model so that it doesn't effectively end up as a huge spaghetti heap of global variables. My personal horror: updating a single database field spurs 20 additional SQL queries creating several new rows in seemingly unrelated tables. Digging in I find this was due to an after_save hook in the database model which created an avalanche of other after_save/after_validation hooks to fire. The worst of it: Asking for how this has come to be I find out that each step of the way was an elegant solution to some code duplication in the controller, some forgotten edge case in the UI, some bug in the business logic. Basically ending up with extremely complex control flows is the default.
So of course, if your code has next to no isolation, batching up queries produces incalculable risks.
/rant, sorry.