Seven years ago I bought what I thought was a late plum variety (Czar) to go with my Opal, an early variety. I got it from Aldi and it was described as being on a dwarf rootstock. It grew strong and healthy to nearly 5 metres but showed no signs of fruiting till last year when a few small 'plums' appeared which I just ignored. This year it has given a good crop but they are NOT plums. Turns out they are damsons and the variety looks to be Shropshire Prune. Anyway, they've now been picked and are on their way to becoming delicious damson jam. I've heard it said that buying cheap fruit trees can be a bit of a lottery but I'm pleased to say this one turned out to be a winner for me.
The lottery as you describe it may have cost me £100 in wasted plum tree investment, but the three Cherry trees at the same modest price of £25-30 may turn out to be the joy of the garden if their fruit is as perfect as it was this year, even quite close to a busy road.
Fruit tree buying is a lottery, whatever you do. I've got two apple trees to replace the Old Dorset trees 1860s variety, and i am glad to say they are both doing well. It amazes me how completely different apples can taste while still posing as apples.
The biclonal pear which lost its mate, continues to fruit by self pollination for many years now, and is an excellent ,and again perfect crop, in mid October.
Lottery? Weak gamble indeed!