[Nevmavrick]

Filler is one of the things that can make shooting interesting. It's one more thing to experiment on.
Those that shoot to "hear the noise" are missing a lot. When you handload, there are more options. I started loading when I was 12yo and told my mom that I was doing it to save money so I could shoot more... now at 63yo I shoot so I can load more, lol.
IMHO, the first video, loading .357 with Clays, is a total waste of time. It prolly won't hurt anything, but has no value. Clays, and similar fast powders, in small cases, and the .357 is a small case in the scheme of things, do not have any difficulty igniting, so the position of the powder won't affect anything. You can load more ammo by not inserting something that doesn't DO anything.
In the second video, ammosmith's, he's using an '06 and Unique. There MAY be some need there. There's a MUCH larger case, and very little more powder. I personally don't think it's that necessary, but think it would be a good idea for you to try it out on your own, to prove it one way or the other.
When I was MUCH younger, I read a lot of Townsend Whelen, and he was a hunter. He liked and had the opportunity to hunt all over the western hemisphere with just what was on his back. A pack, and a rifle had to last him the 3-days to a month that he was in the field. Depending on where he was, and how long he was gone, he would vary the gear. In Panama,ie he carried a Springfield, military issue, and some of his own handloads. Whenever he carried a large rifle, he'd carry a few "mid-range" that were loaded with either "pulled" fmjs, or cast. Then the powder would be pistol powder loaded to the 1200-1800fps range. The idea was to live off the land to some extent, and at 25-50yds he'd attempt to decapitate a bird(no, we're nor talking wren...)or small mammal. That became MY goal when I buy and shoot a "new" rifle, or some pistols. It just adds to the shooting, whether I ever actually do it or not. Whenever I go into the field, I drop a couple of those "mid-range" shells into my pocket.
In the larger rifles, the shells may be carried for several years, off and on, before being used. MOST have a filler, I use Polifil also, but there are several products. The point being, will powder migrate around the filler? (The answer...prolly not)
The tests you need to try are....How much accuracy do you get from the load? Is there an improvement/decrease with the filler? Is there a pressure difference with the filler? Are some powders more sensitive than others? All of these questions are to be answered by YOU.
The loads you'll start with are so low, you'll not be in danger. IOW, 12-15gr of Unique in an '06-.270 etc are going to be in the 10K range in a rifle that is made to be used in the 60K range. Pay attention, and don't stick a bullet. It's a pain to get out, but not dangerous IF YOU DON'T FIRE ANOTHER ROUND BEHIND IT!!!
I shoot ALL my tests over the chronograph, and have for over 30years, as they are tests. All information is appreciated, and the more at any given time, the better I like it.
The calibers I've tested are .300Savage, .308Winchester, .30/30, .30/06, .300WinMag, .25/35 Improved, a wildcat similar to the .250Savage Improved, .257Bob, .375/348 Improved, and .222. The powders vary from Bullseye, to 4895(both of them)
Have fun,
Gene