In that I give you all the reason. The problem here is not its manual detonation, because it is something difficult to master and, as you say, it is only effective in the medium range. The problem lies in his direct shots:
Petrified Cactus:
manual detonation- 32
43 - direct damage
49 - head injury
Electric pea:
manual detonation- 27
36 - direct damage
Capitan canyon:
manual detonation- 32
43 - direct damage
49 - head injury
As you can see, the direct damages are very big, more than any other variant of these characters. If anything GW2 administrators have realized is that a character can not be good at everything. And that's the detail with these variants, or you're good with manual detonation or with critical hits, not both.
And again, these characters are the ones I use the most (precisely because of this damage) and I daresay they are my favorites, but I agree with the community that they are OP.