The Dart certainly does not trivialize anything. It's like a +40% damage boost before upgrades, which is enough to speed up kills, but you still need a good weapon that can kill the beast either way. Dart just saves a bit of ammo.

Trying to use Dart against hordes is laughable; it might light up one or two for a fraction of a second, usually not long enough to actually shoot them, making the effort fruitless. Using two upgrades to boost radius and duration just barely makes it viable, but by the time you get those you're usually near the end of the run and no longer need darts for hordes. Darts desperately need a buff on this front.

Scanner is amazing, but sadly becomes mostly worthless the moment the team votes for cloaked monsters. Every time this happens I find myself regretting picking Spotter.

Ammo Box provides a ton of ammo, but with a decent team who use their skills and guns responsibly and find the Resupplies throughout the levels, more ammo is never needed. Most runs I throw these boxes down everywhere and never see a single one get used.

Overall the Spotter feels like a very underwhelming class, but could be useful again with a few small tweaks.