This is my second year using 5 gal buckets for tomatoes. Drilled a drainage hole about 2” from bottom and did the self wicking reservoir thing. Last year we used only the reservoir and this year we’ve been watering from the top and haven’t found much difference honestly.
In theory, if you only use the bottom water reservoir, the roots will grow long and deep to reach the water in the bottom. Long healthy roots should result in bigger and more productive plants. If you top water only, then the roots find the water close to the surface and don't reach down deep to drink. So you end up with a plant with surface roots and not able to produce as much compared to a bottom fed plant. That is the theory, from what I understand.
Like you, I don't know if I can swear to much of a difference, either. However, most summers I only have to refill my bottom water reservoir maybe 3X in my elevated sub irrigated planter. Since the planters are outside, they also get top watered every time it rains outside. Having to refill the reservoir only once a month is a great time and labor saver for me. I cut a strip of swimming noodle as a water level indicator in the fill tube and that ensures I never run the elevated planter dry, reducing stress on the plants.
Last year, we had a drought with only a little rainfall early in the year and then again right before winter (but after the growing season). Almost all my plants in my main garden without running water dried up and died. Only my plants in my hügelkultur raised beds out in the main garden lived long enough to give some produce. I think those plants in the hügelkultur raised beds survived because what water I could give the plants was soaked up into the wood debris down at the bottom of the beds. My regular raised beds were dry and the soil was pretty much dust by mid-summer. Again, we had no rainfall last summer.
I built some new hügelkultur raised beds up closer to the house where I have running water. I put a sprinkler on them for about 10-15 minutes every other day, and those plants exploded with produce. Again, I think the wood debris in the bottom of the hügelkultur beds soaked up and retained any water not used by plant at the time of watering.
Until last summer with our terrible drought, I too never saw much of a difference between my hügelkultur raised beds and my normal raised beds. If we have a normal summer rainfall, all my plants do well. However, the drought last summer really showed me how much better the hügelkultur and sub-irrigated planter concepts are for growing plants if you can't top water the plants with a sprinkler.
A couple years ago, pre-COVID, I was at a gardening show where some guy was showing his bucket system for growing tomatoes. He had all his buckets sitting on a level foundation, and all buckets were connected together with a hose from one bucket to the next. At the front of this system was one fill bucket where you filled it up with water from a hose, and it automatically filled the reservoirs on all the other buckets. If I ever get into bucket planting, that is a system I would consider using.