Yes
You've already listed the related rules. Yes, additional damage dice are doubled on a critical hit. Yes, that means you can choose to smite when you see you've got a critical hit. Yes, it's pretty good.
From What are the most common damage types per spell level?, there are 14 spells that deal radiant damage, making it the 5th most common spell damage type. These are predominately cleric and paladin spells. There is at least one for every spell level except 7th and 9th so they are available early and often. Radiant damage is a good choice because few creatures are resistant or immune to this type of damage unlike, say, fire which is both the most common spell damage type and the most resisted.
That said, your question is highly dependent on how you use undead in your game. If the players never fight them then obviously it will have no effect whatsoever. If the campaign is a zombie apocalypse and that's all they fight then it will have a much greater effect. How much you telegraph what they will be facing will also be a big factor - clerics and paladins with foreknowledge can optimise their spell selection. If this telegraphing extends to pre-character creation then players may be drawn to paladin and light-domain clerics, this is not necessarily a bad thing.
As to the effect of vulnerability on CR the DMG has this to say (p. 277):
Vulnerabilities don't significantly affect a monster's challenge rating, unless a monster has vulnerabilities to multiple damage types that are prevalent, especially bludgeoning, piercing, and slashing.
This makes sense, if 1 (or 2) PCs in a party of 4 do double damage on a successful hit the effect is as if there was 1 (or 2) more PCs making attacks but there are still only 4 copping damage. The creatures will be down 1/4 to 1/2 as quick but most creatures hand out the bulk of their damage in the first couple of rounds anyway.
Of course, you are the DM, if you slip 1 more zombie into the pack or give the lich 25% more hp the effect of radiant vulnerability will be effectively cancelled out but the PC who picked paladin still gets to feel clever.
Best Answer
This is an extremely potent nerf to rogues
As someone who is currently playing an Assassin-subclass rogue, that class is balanced around dealing moderate amounts of damage normally - not having the same utility as Thief-subclass Rogues in that they cannot ignore the restrictions on magic items like a Thief can, but, in exchange, dealing huge first-turn NOVA damage.
A 17th-level Assassin can use the Death Strike feature:
This is designed to stack with the Assassinate feature:
So that an Assassin rogue could deal what is essentially 4 times as much damage on a surprised target. That is (supposed to be) their ultimate class feature.
By comparison, a Thief-subclass Rogue gets to take 2 turns on the first round of combat with the Thief's Reflexes feature:
Both are designed around the inclusion of Sneak Attack Dice in the dice doubled for a critical hit. However, if you remove Sneak Attack Dice from the effects of the Critical, one of these is much more impacted than the other:
The Assassin Rogue essentially deals 4x weapon damage and only 2x Sneak-Attack damage, and, if your DM is particularly mean-spirited, then they might not even allow the 2x Sneak Attack damage from a failed saving throw for Death Strike. What goes from a spectacular first-turn feature for the Assassin turns into a mediocre feature that can only be used on the first turn of combat. The Thief rogue, however, still gets to perform two full turns on the first round of combat, so it's not nearly as much of a nerf to them.
And the Thief subclass can still use any magic item regardless of race, class, or level restrictions with their Use Magic Device feature:
And, if you compare this to fighters, who get spectacular damage regeneration features (Champion), multiple-attack features (Eldritch Knight), and going so far as to ignore falling unconscious and interrupt the current turn when dropping to 0 HP without dying outright (Samurai, from Xanathar's Guide to Everything).
In the end, combined with the effects in this question about removing extra damage for critical hits on spell attacks, it strongly seems like this DM is trying to railroad your party into playing Fighters, Barbarians, and Monks.