When it comes to frames I seem to favor a few charateristics. Keep in mind my focus on making my copters have all started photography/videography. Over the past few years I have made more then 7 types of multirotors, everything from a quad, x8, few different types of hexa, and an octo. Out of all of them the one I liked best in flying charateristic was my x8 then quad.
So as stated I think one of the bigest things you want to concentrate on when making or buying a copter is weight, and the material they are made out of. I tend to stay with carbon fiber frames, although they are more expensive, for the cost and strength I think they are where any one looking to make a copter gain great benifits.
Another charateristics I find very inportant in a frame is rigidity. This could be the most important out of them all and there are many frames out there that lack this charateristic. Many frames on the market fail in this department, even though when it comes to photography they are suffecient, it when you try to do video they become unforgiving. If the boom have any flex then it will definately translate into the center and result in vibration, we all have see the jello effect, this is the effect that when looking at recorded video you will see odd waves and warping.
This is caused from the frequency of the vibration is much faster then what a camera records at. This result is because the camera is moved from frame to frame much faster then its stored in memory, so each frame, from top to bottom are not the same through out, giving a wrapped look.
This was one of the biggest problems I chased for about two years, in fact I did everything I could to get around the vibration issue, each day I would fly my copter, look at the video, break down my current gimbal mount, and rebuild a new one. I kinda felt like the Wiley coyote with an ACME kit.
There are many things that can contribute to this problem like out of balenced props, out balenced motor bell, loss bearing, bent motor staft, loose motor mounts, and more. Eventhough all the movement and vibrations are produced out at the end of the boom in the motor mount area. Its the rigidity of the boom itself that translate this charateristic to the center of the copter.
When it come to flex it not just in the boom itself you have to worry about being the conduit for the vibration, I would say there is three sections here, one being the motor mount and its connection to the boom, the boom, and the frame where the boom connects to the to it.
If you have any problems in these areas it will show up in the battle of suring up your gimbal. So it very inportant to make sure you have a rigid boom in all areas, if you can do that then you are well on your way for having good video and photographs.
I would say in the frame center hub it is very inportant to have a deep connection from the boom into the frame. in come case you will see one connection point where the boom has no depth and with not depth there is not strength. Its like if you took a baseball bat gripped it like you were going to hit, now instead of swinging you hold it out straight, and have someone push down on the bat, most likely it would take a lot less presure to push the bat down, then if you did the same experment holding a hockey stick out with a hockey grip. Because with the hockey stick your hands are seperated or you have two connection point not on top of each other, this will give you more leverage.
This is the same thing you need with a boom coming into the center, I have found that if you have two or more connection points with some seperation then you will get a very good result.
After a few years looking for a frame with the characteristics explained, I finally found a few in my price range and purchased them. I will say I have not been disappointed and because of what I learned while testing and rebuilding my copters constantly trying to keep the vibration monster down to a bay. I was able to learn what was the key reason that cause my fustrations.
Just remember when you are building your copters that rigidity and weight are very important. If you lack in any of these categories you will be taking away from your products final result.