Sure, it targets Constitution. If you're thinking about it being ineffective against enemies with PC classes, then sure, but that's an atypical opponent. There are plenty of humanoids that don't have great Con saves, and relatively few monsters or NPCs have proficiency in Con saves (some classes of monsters do, like Devils and stuff, but they also usually have Magic Resistance, which sucks all around for casters). Of the NPCs and humanoid monsters (all of which are usually going to be using weapons and are the most often encountered) listed in the DM Basic Rules, only one NPC has a CON save of +4 or higher (the Knight) and none of the humanoids do. Even getting into other monsters, there are very few that add proficiency to their CON saves, they're just rolling straight stats. A minotaur, for instance, only has a +3 to that save, while you are likely to have a DC of 14 or higher by level 4 or so.
The rider effect is, IMO, the best generally available rider status effect offered by the various cantrips. Unable to heal, taking away their reaction, and slowing them are all pretty situational, albeit useful in the right circumstances.
So yeah, I think that
Frostbite is, in most circumstances, one of the more powerful cantrips right out of the box. Sure, one could scour message boards and reddit to copy questionable tricks with other cantrips that may make them seem more powerful, but a lot of that is up to DM permissiveness. Even with its limitations concerning niche cases like monks and more general cases like creatures with natural weapons, I think
frostbite is universally more useful.
ETA: You know what, I'm going to retract one of my thoughts. I think monster natural weapon attacks (bite, claw, etc.) count as weapon attacks given the verbiage from the Monster Manual:
MM said:
The most common actions that a monster will take in combat are melee and ranged attacks. These can be spell attacks or weapon attacks, where the "weapon" might be a manufactured item or a natural weapon, such as a claw or tail spike.
So, I do think that
frostbite, per RAW, affects monster natural weapons, but not a monk's unarmed strike, since it is still an unarmed strike (and, thus, not a weapon under 5E rules). That makes
frostbite generally more useful.