I personally think that was the biggest mistake they could have made.
I too was excited for MNT, especially since their office is near me. The case looks great, the grades are very respectable, and critiqued really well, absolutely nothing is given. Clear plastic, great logo and name, but they couldn't get the sleeve thing right!
That one issue got me from submitting 60 cards, to probably 5.
My first card graded from MNT was a high valued card. I wanted protection first and foremost, not having to worry about it ever again like a card in a BGS case. Except, it moves around. The ridges are hard plastic, so damage is possible.
I emailed them about this, and they replied back saying they've graded 1000 cards with no issues. However, the issue is in the future, not in the present.
I was also told putting the card in a sleeve would damage the card? Cause their standard holders aren't made for cards in sleeves, unless their pokemon cards.
So, I think they tried going with the one size fits all approach instead of focusing on sports cards... and that will probably be their downfall. No sleeves in their cases. Something that seems very solvable.
Nonetheless, Ill still be using their service here and there for cards that aren't too valuable, because trust me they do look better than BGS cases in person.
My issue with MNT is the cards are not encased in a sleeve like BGS. It's the same as PSA. The card can "move" just like PSA inside the graded case.