For most of the career, you'd have a 19 crit on your Rogue, doubling your chance to crit. But while a big blowout critical sounds pretty awesome, it's still a pretty small chance per attack; even with advantage or dual wielding to roll multiple d20's in a turn to get that attack. So even if we say "ok, your level 3 Rogue can now crit 20% of the time" (19-20 rolling two dice, which I don't think is actually 20% but you can talk to some maths guy), all that means is, 20 attacks out of 100 will get an extra xd6 damage. If you're chucking, say, 3d6 sneak attack and using a d6 weapon, that's only 14 more damage on average.
Which can be great in the moment, depending on what you're attacking; a major threat? A monster with most of it's hit points? Fantastic. But it could also be a minor enemy or someone who was brought down to 5 hit points and ANY attack could kill it.
And even if none of that damage is wasted, 20% of 14 is...not even 3 points added to the average damage of this hypothetical Rogue.
Champion on a Paladin has the same issue; you will roll a crit more often but you can't control when it happens. That's why I shake my head at crit fisher Paladins, because they often don't smite when they should, and sometimes when they do crit, the extra damage is meaningless.
Now I'm sure Champion would become a popular choice for some classes, because their eyes get large and they think "BIG DAMAGE", but burst damage < consistent damage.