You still can use HA across dissimilar peers, however for connection mirroring you need to have identical platform. This is due to different logic of DAG running on different platforms and different number of tmms
https://techdocs.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip-service-provider-sip-administration-13-1-0/4.html
Do not be confused by "modes". By logic, I mean that on one platform connection goes let say to tmm1, whereas similar connection (src/dst port/ip) on the other platform may go to tmm0. Connection mirroring requires connection hitting the same tmm on both peers.
In case of non-similar peer you will get next log message: 01340004:3: HA Connection detected dissimilar peer
https://support.f5.com/csp/article/K13828