What did they fail to achieve?
The Chantry established a big followers.
The Qun established a big followers.
Both groups refused to convert and brother Burkel died while demonstrating a peaceful demonstration.
Note that the magnitude is different. From the sound of it, the Qun forcibly introduce as well as possibly brainwashed people into believing while Burkel is more like appealing to a person's faith. You're comparing a crusade with the missionary, but that's besides the point here, the point is that those that they reach with their new ideology, they both converted because they found something lacking in their old belief system. Those who are adamant about their belief wouldn't change their belief no matter what regardless of whether it's peaceful conversion or forceful conversion. Those who claimed to be devout could very well be a believer for the sake of believe, They have to believe in it because they have nothing else to turn to, if a new ideal was introduced, some of those devotion could very well switch targets. All I'm saying is sometimes, it's not about how good or bad a certain beliefs are, it's about understanding the psychology and what a person turn to religion or a new ideology. For a short real life example, some of the members of Heaven's Gates cult was not clueless people who were tricked into it, some of the were professionals and well educated, There were a variety of reasons why they turn to a cult, but they weren't being forced into it, those that were forced into it by their loved ones ran away, Those that stay stayed because they found someone in it that they didn't find in their daily life. There are also stories of Americans convert to extremists and homegrown terrorists. In some of these cases, it's not about the morality or being force into anything, it's about whether a person can identify with that idea which could very turn into their new purpose in life.
The difference is that where the Chantry tried to change the beliefs of Orzammar, they were rebuked.
The qunari changed the believes of the Andrastians they converted, to the point where those who were converted would rather die than convert back.
If the Qun was as oppressive as some make it sound, and people they conquered and converted merely claimed to be devout to save their lives, then the Chantry would be hailed as liberators who saved them from the qunari oppression and let them return to the faith of freedom. Instead, the Chantry found that many people who were once devout followers of the Chantry who now rather die than abandon the Qun.
And that's my main point: Despite our claims that the Qun is too oppressive for people to follow and that the Chantry gives so more freedom, many people who are converted in the world of Thedas seem to have a different opinion.