3.5 was edition I played the most. It was a reason why I quit RPGs for nearly a decade because I hated it so much.
Every time I see another meme about how amazing 3.5 Tarrasque is, I remmember how amogn actual 3.5 players Tarrasque was the biggest joke. It was always brought up as definite proof designers have no idea how to make good monster. It was laughably easy to beat. A wizard could casually solo it, the same abilities people now miss in 3.5 amounted to ribbons. It was a laughingstock, forums had 100+ pages discussions how to fix it and general consensus was it';s beyond saving. It was first proof in 3.5 if you cannot use magic you're only good to roll over and die.
I honestly don't know if everyone claiming 3.5 Tarrasque is such a horrifying monster are trying to rewrite history or unintentionally proving what a broken, unplayable pile of garbage 3.5 was, if it's biggest punching bag is actually dangerous in a different, better designed game.
I've heard this line so many times, from virtually every game system. The system you know the best is always the worst. The system you're least familiar with looks genius by comparison.
As I understand it, the Tarrasque isn't intended to be a direct threat to the players so much as a civilization-wide threat that players have to deal with. If you're just running heads-up against the creature, there's a wide basket of indirect effects and clever builds that can kill or disable it. And when Wish/Miracle are on your spell list it isn't an existential threat to a 17+ level party.
But all of that presumes you're coming into contact with a Tarrasque as a known quantity. You're not stumbling on the Tarrasque unexpectedly or dealing with it as the muscle attached to a more magically or socially savvy antagonist. You're not fighting in any bizarre circumstances or unusual conditions. It's not the Tarrasque that's easy, it's the fact that you're on a message board with a pre-defined set of circumstances and a standard level appropriate set of resources to pull from that makes things easy.
An unanticipated introduction to a Tarrasque, particularly one encountered in unfavorable circumstances, can quickly end in a TPK. Players down on spells, caught napping, managing some secondary hindering conditions, or in an enclosed space (the meanest improvement I've seen a DM give to a Tarrasque was simply assigning it a burrow speed) don't have the luxuries of time and distance to prepare themselves. And that's what makes it scary.
But, again, you can say that about any of the Animal/Beast class of monsters. The humble house cat can one-shot a first level wizard if it gets initiative and rolls well. But the wizard wins with a single volley of magic missiles. The Kraken is a trivial encounter if your players can sit up on an 80' tall cliff and fire arrows at it until it drops. Its significantly harder to deal with when it is demolishing the boat under your feet 600 miles off the shore.
Part of the DM's job is to set the stage for high drama. "You see the big baddy waltzing up to you, take ten rounds to prepare" doesn't get you that.