Which feature of GETVPN is a limitation of DMVPN and FlexVPN?

Sequence-number-based replay checking is a standard IPsec feature available in DMVPN and FlexVPN as well, so it is not a differentiating capability unique to GETVPN.

ESP and AH are IPsec encapsulation options supported equally by DMVPN, FlexVPN, and GETVPN, making this a shared trait rather than a GETVPN-exclusive feature.

GETVPN is specifically designed for trusted private WANs such as MPLS and cannot function over the public Internet, whereas DMVPN and FlexVPN support both public and private transports - so this describes a limitation of GETVPN, not of the other two.

GETVPN encrypts packets in-place while preserving the original IP header, meaning the existing underlay routing infrastructure delivers packets without any virtual tunnel interface or overlay protocol. DMVPN and FlexVPN both build mGRE or IPsec tunnel interfaces that require an overlay routing protocol such as EIGRP or BGP to advertise networks across the VPN fabric. Because GETVPN has no tunnel overlay, no overlay routing protocol is needed, which is a capability DMVPN and FlexVPN lack.