With the low cost of some processors, you can do a lot of fun things with a little slave or co-processor.

No, there are no hidden gotchas. It is just a design choice. Using a larger main processor, or adding discrete logic outside, or programmable logic or a second microcontroller. The microcontroller and programmable logic routes have the disadvantages that you get one more chip to program and may get into troubles with field updates. The advantage is of course that you can update the logic. With discrete hardware, you will need to redesign the hardware if your requirements changes.