Let me start with a clarification : BCG Platinion is much more technical than McKinsey Digital. Assignements done by BCG Platinion are rather implementations or IT diagnostics. If you want to do IT or Digital Strategy, you should apply to the BCG Technology Advantage practice.
Regading you question : usually when interviewing for McKinsey Digital or BCG Technology Advantage, you can get 50/50 between generalist cases and IT cases. Platinion may have a higher ratio of IT cases during interviews.
IT cases are very often around IT costs reduction. So, you should have in mind some key ratios / order of magnitude (such as internal and external FTE costs, split between project and run costs, split betweern software and hardware costs).
You may find get cases on Agile or Digital POC as well.
Hope this helps !