I heard that quality issues have been a problem for longer than the past year. What I was told was that Gary hasn't been as involved with nursery op's as he used to be. I think it's a combo of that and the booming housing market + booming economy which have driven up demand.
I think the issue can be partially mitigated by going to zills in person and selecting trees vs using delivery and having them select trees at random to go on the delivery truck. I haven't been up there in probably 4 years, so I'm relying on word of mouth and pictures I see on this forum of little spindly 3 gallon trees.
During the Zill's quality heyday, a typical 3 gallon would have 6 - 7 full flushes (whether branched or unbranched is largely irrelevant -- the point being that they were well grown and ready for pot-up to 7gal), and were barely a year old from seed.
Fortunately mango trees do grow rapidly if dropped into the right environment. But there were some serious bargains back during the economic downturn -- including bearing jaboticabas for under $40
.
Some of the recent trees may be small since demand has been so high following Irma and they aren't staying at the nursery so long.
All of mine were reasonable size and good quality but I bought early this year and heard the demand was very strong. When I was selecting there I went for diameter over all other factors, then if some had branches reasonably high I went for them, height was my last concern. Most trees I planted got tipped the same day I planted them if they hadn't started already.
I could tell none of mine had been in pots too long, all I did was run a knife up and down down vertically and criss-cross along the bottom to find anything circling and there weren't many, only toothpick size or less.