That’s a clickbait title. There’s no issue with the Explorer-as-such, he is a great class, well designed, fills a niche, definitely a nice chap.
There is, however, some issue with his niche protection. Wilderness exploration is one of the core things to do in a game of ACKS. The Explorer with his powers, especially the Evasion -power, is something of a necessity in wilderness exploration. One can find ways around this, like traveling with a large entourage of troops, but if traveling as a small group of adventurers an Explorer is basically mandatory for purposes of evasion, which you definitely want to do at times.
What, then, is the issue? Well, let’s say you want to play a dwarven, elven, Zaharan, whatever game, with everyone a member of one of these races. Now, one will find wilderness exploration difficult. Especially if one plays a game in a world that lacks large settlements of the race. Trying to recruit even a full platoon of dwarven troops is a nontrivial thing in market classes below III, and not easy and automatic even in III. Using human troops instead is potentially unpleasing in such a game, as is being forced to recruit a human Explorer for his evasion if wanting to travel/explore with a small party.
Now, some may argue that this is a feature instead, that those races should not explore new lands or should do so only with an army. To this I say: Pish-Tosh. Wilderness exploration/travel is pretty key in stuff like going to reclaim/find a lost vault, and recruiting an army or a platoon even is an issue. Practically locking away a race from a pillar of gameplay is unpleasing as a design element.
What can be done, then? The creation of classes with powers similar, or the Evasion power, obviously an option. Warden, designed by me, was an attempt at an Explorery dwarf, I do love the class but it does not manage to do the thing. I have a new version/a different class planned for that, not just picking up Evasion but doing something similar. Picking Evasion, obviously, for a hypothetical elven/dwarven/whatever class is the easy mode, but this may be seen as devaluing the Explorer, and it may not be bad that the Explorer remains the best at what he does either.