It absolutely IS.Not being able to full length resize your brass isn't a limitation.
Tell a guy with 2 different .30-06s or .30-30s, and a Lee Loader, that the inability to FL size is not a limitation, and see what they tell you. Odds are that anything fired in one of those is not going to chamber in the other.
...Or the guy with 12 ga shotguns manufactured in 1993 and 1929. Anything fired in the newer shotgun will not chamber in the older shotgun, due to the bases being too large. (Standards became very loose for the 12 ga, over time, due to crappier and crappier plastic hulls being made -- and the Lee Loaders for shotguns don't resize at all)
There are ways of getting around it, but it adds time and energy to the process.
For example, you can label your ammo for the specific firearm it came from.
But... what happens if circumstances push you to slowly start shooting all of your brass from the "tight" chamber in the "loose" chamber, and you end up with nothing left to reload for the "tight" one? -You're hosed, and have to buy more brass.
What happens when you and a friend head out to break a few clays with your shotguns, and he ends up firing 3 boxes of shells in his shotgun, only to find out that nothing fired in his gun will chamber in any of yours? -You're now the not-so-proud owner of 3 boxes of useless hulls.
Not being able to full-length size is not the end of the world, but it is a limitation that comes with the Lee Loaders. It should be considered before the purchase.
For a man with multiple handguns, rifles, or shot guns (especially) in the same chambering, the Lee Loader can be a very poor choice.