1. The very first limitation is: CMS works on Jelly, Service Portal works on Angular JS. So, if a custom page needs to be migrated from CMS to Portal, entire Jelly code (if any) needs a redesign which might need some concrete Angular Knowledge.
2. Some OOB scripts like Client-side UI action do not work on Service Portal, which need to be considered before migration. So buttons which open pop-ups need to be looked upon.
But again, there are more Pros then Cons, plus Angular is quicker with a customer friendly UI, so it has a better future. Migrating from CMS to SP is considered a great move.
Please go through this informative blog by Goran: Has UI Pages a future in ServiceNow?
Let me know if this helps and mark Helpful accordingly.
