I've never worried about Warlock since EB requires multiple attack rolls.
Well, as a bonus action the sequence (using Fire Bolt) becomes TS+FB each round. So, the second round forward the FB has advantage each time. With a 2d10 FB, this bumps average damage from 7.15 to 10.3125 assuming 60% hit probability. That is a 44% bump for a cantrip each round.
Now, let's look at the restrictions:
1. It is concentration, so it can be broken. (This doesn't happen much at my table, but YMMV.)
2. The target must be within 30 feet. (I see this as more significant because it means you can't use TS at decent range.)
3. It requires the bonus action. (Now, a lot of casters have some bonus action stuff, but for Arcane Tricksters this is HUGE because it means no Cunning Action.)
4. It is self only. You target a creature within 30 feet, but you can't have someone else benefit from the spell since it is
your attack roll.
Are those restrictions enough to warrant a 44% bump each round? It sounds like a lot, but it is really barely 3 points in this example. It does prevent other concentration spells, it allows only a cantrip to be cast the same round, so repeated uses means repeated cantrips and not other attack spells.
As a bonus action, it allows the EK to attack each round it is cast as well, but he only benefits on his first attack next turn. I suppose that is significant but not
huge.
You know,
@jaelis, I am coming around. Maybe it
isn't too big of a buff to the spell... I'll have to give it further thought.