That's good to know. I generally aren't burying mine more than 8-10 inches, and I put water walls on mine that heat the ground up around them. I doubt the soil is very warm here either at a deeper depth.I have experimented using deep holes vs trenches. Normally tomato plants do not send roots any further down than about 8" to 10" deep. Here, the deeper you dig the hole, the colder the ground is. The ones that I planted by digging a hole 16" deep so I could have most of the stem buried would "stall" in their growth until the stem in the upper part of the ground grew enough roots to support the plant. The roots in the lower part of the hole eventually died off most likely due to being at too cool of temperatures. When the plants were pulled in the fall the roots were only about 8" deep even though the root ball was initially planted at a depth of 16".
The ones I planted by laying the plant in a shallow trench no deeper than 8" did much better right away and did not go through the stagnant growth period. When those plants were pulled in the fall their roots also were no deeper than 8" but were more numerous and spread much farther out from the main stem.
If your soil is warmer (at least 60°F) at deeper levels than my soil (sand) is then this information may not apply to your case.
This year my tomatoes are going in my raised boxes, I will probably need a step ladder to pick them.