Find a new LBS guy. The one you are working with is giving you advice tailored to what he sells. He is either doing that or he lacks experience to be opening his mouth. Or he is an idiot.
Its not that Tannus Armour and puncture resistant tubes are bad. Use those. But calling Slime/FlatOut scams is just plain ignorant... at best.
FlatOut actually works very well on large holes. Thats its chief difference over and above Slime. It is rated for 1/2" holes and it has sealed a jagged piece of metal that was very nearly that on a tubed tire. When used as a tubeless sealant, and I hit a strip of a half-dozen roofing nails (from some kind of nailgun) it was able to seal the half-dozen holes so I rode home. And that tire was tubeless. FlatOut is a superior tubeless sealant since it can do what Stans and Orange Seal cannot: Seal big holes.
I am in my late 50's and have cardiac issues so walking home pushing a bike would be very very bad. Especially since we just crossed our local record of 64 days over 100 degrees fahrenheit yesterday.
Here is the Bolton test. I know Kyle (the guy in the video) and he's a straight shooter. This test is on the up and up.
More on the subject:
As a daily rider for decades, I have some thoughts on this subject. Mostly along the lines of “no flats allowed, ever”.
talesontwowheels.com
If you want to really bullet-proof your tires, use all three and more. The front tire on my Bullitt has a puncture-guard belt inside of it. Under the tire is Tannus. Under the Tannus is a thornproof tube. Inside the tube is FlatOut.
Here's a thing about Tannus: At higher pressures (higher than fat bike pressures) it flattens out paper-thin. Tannus talks about this and suggests you air down your tires in between rides so it takes longer for this to happen (seriously). Tannus is a product too new in the market to say it will last for X years. Hopefully it will. I've got it in several of my bikes. BUT if I can't fit it in under a tire as I cannot on some bikes, or I can't do a thornproof tube like on my fat bikes, then the default go-to is a tire and FlatOut. That stuff is good enough to make me feel safe. that and an on-board portable compressor.
And of course on a tubeless setup its just the tire with FlatOut inside as a sealant. Those are my enduro bike and my Fat trails cargo bike... the two that get the worst-case use in the middle of nowhere.