For me an example of "joining an evil cause" done right would be in Divinity Original Sin 2 (spoilers for that so don't read if you intend to play it at some point).
Basically there are a ton of quests along the way in which we can do the bad thing, whether by action or inaction. There are a huge amount of such choices throughout the game for reasons of power, greed or selfishness, but they're small choices that don't commit to any morally bad cause.
But then comes along one major choice that truly stands out and that is the choice to join the God-King if we're playing as an Undead ourselves.
In ACT III the party is approached by a random Voidwoken, who until then were perceived as nothing more but ultimate evil with no remorse. Until it is revealed that they are in fact not monsters but the original inhabitants of Rivellon, known as Eternals; who were betrayed, tossed into the Void forever and ended up getting corrupted by it because 7 greedy Eternals in the service of the God-King saw an opportunity to become Gods themselves and betrayed their own race in order to do so.
- By learning that these Voidwoken are in fact betrayed Eternals who just want to take back what is rightfully theirs in order to live as Eternals once more, we now have the understanding of their reasons for doing bad things.
- The next revelation is that we have a chance to help them return into Rivellon so they can once again live as Eternals instead of the corrupted shell of what they used to be. Now we have a reason to make that choice and an incentive.
- And if we choose to help them return to Rivellon by becoming sworn to the God-King, he promises us a lot of great things. Now we have a very powerful motive and a potential gain.
- Ultimately joining the God-King gives us a permanent and incredibly powerful buff for as long as we remain in his service, not to mention other promises related to story. Now we have the reward.
So it is by all means a bad choice, but it has everything to make the bad choice tempting and worthwhile. The understanding, the incentive, the motive and reward are all present. The only thing that might stop me is morality, but all the positives are present.
When it comes to Minthara however... I dare say it's just a badly written encounter in its current state that exists purely for the sake of being evil, since it goes against the purpose of the party's goal, does not progress the plot in any meaningful way nor establish any sort of meaningful incentive for joining her.
It feels awfully lackluster because there are no actual story related motives nor reasons whatsoever behind the decision to join her unless you really just want to go on a senseless murder spree and then do some more by killing her too.
Even if we count deep-undercover infiltration as a story reason, it is a terribly weak foundation for the premise since we are already able to fully infiltrate every single layer of the Absolute's army. Even Minthara and Nere end up being easily deceived as long as we pretend we are following the Absolute. So what is the purpose of joining her and acting like we're undercover when we already are doing that without an issue. So once again.. the choice is just there for the sake of being evil without any meaningful substance.
So it's no wonder that so many players do not care about the evil path when it severely lacks substance. The actual reasons, incentive, potential gain and most of all motive to join Minthara on the raid are not there. It only exists so the player can be evil for the sake of being evil and nothing more.
Story does not progress any further than what we already know we should be doing (which is getting to Moonrise), not to mention we lose everything and gain nothing by it. We lose the entire Grove and the entire Goblin camp... for what. A hint to look for a Drow Orc, who we would without a doubt come across on our path to Moonrise if we go above the surface.
