I was part of a “non-denominational” Bible study in college (by which they really meant “anybody but Catholics”). They kept trying to invite speakers who would come and give emotional testimonies about how God saved them from a life of sin and debauchery and crime and drugs… and the head chaplain kept saying, “NO,” because, well, we were at the U.S. Naval Academy and none of us had that kind of history! When I pointed that out at a leadership meeting, nobody got it. “But it’s a powerful testimony!” That’s great, but it has nearly nothing to do with midshipmen, who generally are hard-working, non-criminal, fairly responsible people.
Later, with a Bible study in Virginia associated with the same organization, the leader encouraged everyone to write and practice their testimonies. (My husband told about how he chose to become Catholic.

) Most of the testimonies tended to have way too much personal detail and a hint of sensationalism. We were told to write them as a sort of “hook” to get people interested in talking about Christ.
Kimberly Hahn told a story that, for her, helped explain Mary’s Immaculate Conception, but it also touches on “testimonies” and their tendency to fall into a sort of spiritual one-upsmanship:
Kimberly was at a Bible study with a friend, the son of a pastor (aka PK, Preacher’s Kid, no disrespect meant, just to give him a name). The PK was a great guy, very devout, straight laced, etc. Everyone was giving their testimonies: “Jesus saved me from lying and gossip!” Next: “Well, Jesus saved me from lying, gossip, and smoking!” Then: “Well, Jesus saved ME from all that AND sexual immorality!”
Finally, PK stood up and started, “Well, Jesus saved me from lying and gossip and smoking and sexual immorality and alcohol addiction and …” Kimberly looked at him in shock. PK had never had any of these problems! What the heck was he doing, just trying to show everyone up?
Then he finished, “… and Jesus saved me from all of that by giving me good, solid, Christian parents; a wonderful church; and keeping me with good friends who held me accountable.”
Later in life, Kimberly cited it as a way of explaining how Mary is still saved by Jesus, just as a preventive rather than as a remedial effect like the rest of us. However, she also gave an example of how rare that kind of “long, slow obedience in the same direction” testimony can be.