The article explains the limitations of Express Path/service offload

Express Path does not support:

• Features
  • Transparent Mode
  • Multicast session with more than one fan-out
  • Fragmented packets
  • IPsec VPN
  • Different MTU size values
  • J-Flow
  • Flexible VLAN tagging
• Application Layer Gateway (ALG) data traffic:
  • DNS
  • IKE and ESP
  • PPTP
  • SQL-NET
• IPv6
  • NAT
  • Transparent mode
  • Different MTU size values
  • Class of Service (CoS) on egress interfaces

Express path and packet offloading doesn't work when you use firewall filters to direct traffic into a virtual router,

If you enable Express Path on a device operating in chassis cluster mode:

• You cannot configure asymmetric I/O cards (IOC).
• If a child link from the LACP-enabled reth interface goes down, all traffic on this link is distributed to other active child links of the interface. If the child link comes up and rejoins the reth interface, then the existing traffic or sessions are not redistributed over this newly rejoined active child link. New sessions traverse through this link.
• If a new child link is added to the LACP-enabled reth interface, then the existing traffic or sessions are not redistributed over this new child link. New sessions traverse this link.

Refer this document for more details Express Path Overview