Rally RS200 on Kickr Core: 10% difference

Why do you think there is a problem with Rally? :D Anyway, I would just ignore the integrated PM and rely only to one power source - Rally pedals for indoor and outdoor riding.

If the Rallys are correct then my Kickr is far off... So either way one thing is not working as intended

I would do a couple of really hard sprints and test again.  

4888723 the Core is an inaccurate trainer (even if it advertised as accurate ...) by deafult, do an advanced spindown after a proper 10 min warmup (10mins at 150w). Advanced spindown: a secret menu in the Wahoo app, tap 10x times at Core trainer icon, and it will be shown, after advanced spindown, compare with Rallys again. I`m waiting for your result!

Rally pedals are less sensitive to installation torque than older Vector 1 / 2 spindles but the spindle tightness in the cranks still matters, at some point. The pedals work by measuring the flex in the spindles as you pedal. If the spindles are loose in the cranks, they don't flex the same way as they would when they are tight. Additionally, under-tightened spindles will eventually damage the cranks because the thread surfaces end up being able to rub on each other instead of being locked in place. So - if nothing else, tighter is better than looser.

Secondarily, if you got an FTP of 196W in the Sufferfest's protocol then doing 200W for an hour would be pretty much right on (or over) your FTP level, and you should have felt completely exhausted for some time afterward. Did your RPE match up with the numbers coming out of your trainer?

I tightened them, Cali rated everything again and now it's way more close between the rally and the Kickr.

My FTP instantly got changed in Zwift to 210W. But no I wasn't completely done after the hour. It felt like giving it everything with surviving :D

I've run comparisons of my RallyXC200s against my KICKR Core and TACX Neo. They match the Core well. The Neo reads lower. I've found that tightening them snug and a couple of rides tends to bring them closer. With that said, I use them as my primary and only source of power now for training on TrainerRoad and outside.

I do experience an occasional dropout over BLE to my iPad on TrainerRoad but I suspect that's due to interference at the location that it happens at. They've been pretty solid over Ant+. I calibrate prior to every ride and reset Install Angles each time I take them off and reinstall them. I can post some of the comparative data I've matched up on the DC Analyzer tool if interested. 

Where can you set the angle? Never got ask by the app or my Fenix...

The installation angles are automatically set and continuously updated, without any direct action required by you. However, if you have re-installed the pedals and you want to speed up this process (from about one minute to less than 10 seconds) then you can explicitly clear the install angles, if your head unit or watch has a menu item for doing so.

Yup..they do. Ray recommends resetting them when you switch bikes.  Garmin Rally Power Meter In-Depth Review covers his suggestion. I only mentioned it as I switch my pedals between bikes and have done it.