That's a tough question. Our sons served missions (last one got back in 2015, so fairly recently), they finished them, and they have remained active in the Church. They learned a LOT on their missions, but most of it was not what many might think. They suffered quite a bit while on their missions, because despite what some idealists want to think/believe - it really IS all about the numbers. Their whole missions revolved around numbers and how many people they could baptize. The pressure was insane - about as non-Christ-like as you can imagine - and Sec.121 had, seemingly, never been read by or understood by most, including mission presidents. Other negatives were - getting things stolen quite often by other missionaries; bullying to the extreme if you were at all different than whatever the current "norm" was in any particular area; seriously bad influences (mainly other missionaries - I won't go into detail), etc.
Our sons were exposed to some pretty heinous things while serving their missions - not so much by the world or people they were teaching, but by other missionaries, members, and especially one of their mission presidents (a few months after my son got home, his recently released mission president was called as a Seventy in GenCon, and I literally could not raise my hand to sustain him - given what I knew of his true character from interactions I had with him while my son was in his mission).
Of the 11 boys from the two wards in our town who have left on missions in the last 4 years, only THREE have stayed the whole 2 years. ALL of the other 8 boys came home early. ALL of the boys who left soon after their 18th birthday (like within 2 months of turning 18 or graduating from high school) came home early - every single one of them. How's that for a statistic. The 3 boys who stayed the whole 2 years waited till they were almost 19 to go (one was 19 when he left).
I was in the FB groups of our son's missions (where parents, mostly moms, would post about their child's experiences, etc.), and the VAST majority of missionaries who came home did so because they were forced to go on their missions in the first place by parents who, apparently, didn't listen very well to the entirety of Pres. Monson's words on the matter (quoted elsewhere on this thread) - and felt that in order to "follow the prophet" or appear righteous, they had to force their barely 18 year old child to go live with strangers in a different part of the world, some speaking a foreign language (that of course they didn't know how to speak), etc. In other words, the boys (because that's what they are) came home early because they didn't want to go in the first place (at least not as early as they were forced to), and they couldn't hack it. The latest one from our ward who came home early (after only being out just shy of 2 months), is the son of the 1st counselor in our stake presidency - and yes, his parents forced him to go right after he graduated from high school. He had made it well-known to all, quite publicly, that he had zero desire to serve a mission, but his high profile (stake presidency) daddy and old-Utah-family mommy wouldn't hear of it, and literally forced him to go. And that exact same situation applies to every single one of the boys who came home early.
Anyway, what a deal - these poor boys have to live with the stigma (which, of course, shouldn't exist, but let's be real, it does exist) of "coming home early", when they didn't want to go in the first place but were forced to by parents. Sad situation. Our boys learned lots of good things, too; I hasten (had to use the word at least once) to add - but I would not say their missions had a net positive affect on them. I will say they are still active and thriving in the Church and gospel, no thanks to having served missions.