Any gotchas when migrating NetScaler from ESXi to KVM? According to the documentation live migration is not supported, which I guess we can live with - but anything else? Does it run well?

Make the KVM the secondary to the ESX primary. Use STAYPRIMARY and STAYSECONDARY as well as ensuring you have a good backup. Match the NICS, VLANS, and speeds if possible. Once they are in HA, stable, and the configuration has replicated, remove the STAYs and failover. Test thoroughly! Then replace the VMware secondary with a KVM secondary.

Generally saying: HA works very well over different platforms, even if it is absolutely unsupported. While I would not use HA VPX/hardware in production, I frequently use it for migration. By the way, I have a customer using hardware (STAY_PRIMARY) and VPX in production. He had a virtual HA pair, but it turned out to be unable to handle the amount of SSL transactions per second, so he had to switch to hardware. However, he didn't have enough budget for 2 boxes, so he bought just one and kept the other one in case the hardware crashed. It has worked for ages, several firmware versions since 12.5.