I have both the 100-400 L and just picked up a Tamron 18-400 as a walkabout lens. I still have a week in which I could return it with no restocking fees.
I am taking a lot of shot with the 18-400 to see if I should keep it. It seemed to me to be a bit short at the far end, i.e. 400mm so I decided to test the FOV against my 100-400 L IS II. Also compared to my 55-250 STM
the 55-250 and the Tamron seem to be about the same FOV, but the 100-400 is different by quite a bit.
on the test against the 55-250 there is a very slight difference and it could be where I estimated the 250mm spot on the Tamron was.
But with the 100-400 there was no doubt that when I zoomed all the way out on both the Canon was showing the same FOV of the Tamron at 400mm but the Canon was at 300mm and then the 300mm was 250mm on the Canon.
Tamron Canon 100-400
Tamron Canon 55-250
One thing about the Tamron, and helps as a walkabout is that the close focus distance is about 18 inches. In reviews I have heard that the Tamron 16-300 and the older 18-300 are not quite as sharp as the 18-400.
Sharpness, it is really dependent on holding the lens steady and the subject not moving. I have gotten some good sharpness but also alot of hand held shots seem to be a bit soft.
I had thought about the Canon 28-300 and still agonizing over that one, but it is over a pound heavier and 4 times the price! Also close focus distance is 2 1/2 feet vs 18 inches
AND at 28mm, with my 80D that is 44mm equivalent vs 28mm equivalent.
Here is a Cardinal I shot Tamron 18-400 at 400mm ISO 1000 1/250 f/5.6 Cropped and sharpened a bit in Photoshop