There is no down fall if done correctly. IMHO it is required if you want to increase the flex compared to leaf springs. By cutting out half of the frame rail you are decreasing strength significantly so you must reinforce the side that you keep and make sure your coil box is strong. I made mine out of 1/4 plate and I reinforced the inner frame rail wall on both sides (this requires dropping the gas tank which I had to do anyways to make new shock mounts). The inner frame wall is now 3/8" thick in this area. I have frame stiffeners, now I just want to plate the rails up to the coil buckets, I was waiting because this requires removing the front leaf mount.
I made mine very similar to Clayton's coil conversion kit as far as the upper coil buckets and mounting.
Here is a picture of mine, notice that I also tied it in to the floor and wheel well.
That was my though stiffer spring rate will keep the weight of the body more stable reducing body roll when mounted inside, but should flex the same because the axle will leverage the, more, same concept as using a cheater bar
So many people to thank here's a start to the list
Spidertrax, Yukon, rustys, warn, branik, PSC, advance adapters, Bad wheels, interco, essentially off-road. And many more
Most of all thanks too Ian Johnson and the xtreme off-road crew for all their work, and 4 wheel to heal for their part in it too.
Ahhh.... I did not understand the question I guess, I thought you meant mounting under the frame rail (like Treks kit) vs inside the frame rail (like Clayton kit). If you are talking about mounting the spring inboard of the frame rail then yes I would agree with the stiffer spring rate.
I imagine that mounting them inboard of the frame rail would cause more body roll, so you would need to keep your sway bar (most people remove the rear for coil conversion). This is because the countering force when turning is closer to the center. Take this to the extreme...think about a very narrow spring stance the counter balance from the body will not effectively keep the body straight as it would for a wider spring stance.