I've done quite a few Mentor matches now, but the game still suffers from the same problems of high MMR players raising the difficulty too high for newer players to cope well.
As an example, I had a Mentor match that was set at 47% difficulty, even though we had a level one player there. Needless to say, he wasn't having a very good time of it, and would generally die a lot to regular enemies unless we watched out for him. A number of other Mentor matches I've played, were also set around the 40% difficulty range, which I would say is too high for beginner players.
I've also had Antagonists invade in Mentor matches, and they were always a far higher level than all the newer players. I had quite an annoying match on In Medias Res, where there was a level 150 Patient/Reborn Doldren who would just teleport around and one Strike down the lower level players. The level 2 Alicia ended up quitting after a few deaths, and the level 8 Lycus was basically a free kill for the Doldren. It was basically just me and another player having to do everything, whilst slowly losing lives due to the Antagonist picking off the Lycus over and over. I also had to fight a level 200 Shepherd Rak, who would generally just spawn next to the lower level players and one shot them, so the whole match became a glorified escort mission, which was hardly fun for anyone involved.
It would be better if the difficulty was capped to a very low level, perhaps around 20-25% difficulty at most, regardless of what the average MMR of the team is. Most newer players tend to Strike or Grapple spam their way through enemies, and that just doesn't work at higher difficulties. A lower difficulty would at least allow them some time to experiment with the melee system without dying over and over. Beginner players also don't have much, or any, weapon upgrades, so it would be better for them to go up against lower health enemies that don't require as many shots.
I don't think Antagonists should really be allowed into Mentor matches. Newer players just end up as free kills, and most of the time they just end up leaving. It's also frustrating on my end having to play babysitter. In cases where a low level Antagonist goes up against much higher level Raiders, they'd probably see it as being unfair too.
If there were no Antagonists and the difficulty was capped, I don't think older players would mind doing Mentor matches as much, since they would probably be able to carry without too much difficulty.
Of course, it's difficult to communicate with players at the moment, so that's still one of the more glaring issues of Mentor matches.