The answer is it depends. There are many factors you have to consider, and some challenges you have to overcome like everything else.
I have used harmonic drives with long focal length systems extensively, with some decent results. The longest focal length I used was my C11 on the AM5 at about 2065mm. It is not a trivial task but doable. Currently I routinely use my AG Optical FA12 with my HAE69ec. This is a special case as this harmonic mount is equipped with a high precision encoder on the RA axis which helps. I am not aware of any other harmonic mounts with high precision encoders currently other than the iOptron variants from the HAEec versions, but I could be wrong.
The first challenge as far as guiding is using an OAG. When using such a long focal length on a reflector, an OAG is preferred. The issue with harmonic drives is that they need frequent corrections to counteract the large periodic error, which means short exposures, 0.5-1.5 seconds. I often used 1s. The problem with such short exposures is at long focal lengths, it will be harder to find guide stars, especially during galaxy season. So it is important to use a large prism OAG with a large sensor guide camera. Large pixels preferable. I found the combination of the 174mm along with the ZWO OAG-L to work well for that purpose to maximize the chances of finding multiple stars suitable for guiding. Personally I never went past f/7, so at f/10 this can be even more challenging.
Then comes the issue of size. The weight is not the only factor that comes into play here. How far the center of mass of the OTA is from the RA axis matters. If you get two systems, with the same weight (lets say 30lbs). One is a 6 inches in diameter while the other is 10, the mount will have a much harder time moving the 10inch system compared to the 6 inch system, despite having the same weight. Unfortunately not all mount manufacturers explain this in their specifications.
With that being said, you don't need to limit yourself to half the posted payload limit. But if you are using a 10 inch or 11 inch SCT, probably keep your payload 10lbs away from the maximum limit to take into account the OTA center from the RA axis.
Finally it also depends on your location and seeing conditions. With such frequent exposures, you will be heavily dependent on your seeing conditions. Multistar guiding definitely helps, but you may not always be able to get more than 2-3 stars, and sometimes 1 star that is suitable for guiding. Things may have worked well for me because I have good seeing conditions especially in the summer with my AM5. But if your seeing conditions are poor, your guiding will suffer, and so will your images.
Hope this helps.
