I am in the progress of doing ISSU upgrades on some nexus 7710 chassis' with dual supervisor modules. I set a ping test that to traverse across the switch shortly before the upgrade and experienced about 15-20 seconds of lost pings. Is this test scenario a good one and does this amount of downtime seem acceptable? Is cisco's marketing on this feature just bs? I see the same behavior for a supervisor switchover as well.
The best tip I can give for anything involving ISSU on 7K's is to read- and re-read- the release notes. There are very specific versions that are supported as origin/destination and there are also some features that are going to work better with ISSU than others. The other caveat to consider is that in some instances there may be requirements for firmware or EPLD upgrades that could create a brief outage (again - usually mentioned in release notes).
In general I can tell you that the ISSU process can be truly hitless but that you need to stick within the limits of what QA has specifically validated.
I found the issue. The interfaces of the 7K we were upgrading didn't have the 'no ip redirects' command on. This caused a drop in BFD between the neighboring router we were testing from and also dropped our EIGRP neighborship. This list has similar information: https://lists.gt.net/cisco/nsp/141379