I’m not convinced I would ever encourage your average amateur eventer to dive into buying weanlings or really young horses. Ironically most of the amateurs I know who do this aren’t tight for funds and aren’t doing it to save a bunch of money. Even if the risk part of it works out really young horses are not the financial savings most people think they are. You can expect to pay $10K +/- on top quality purpose bred foal. Then three and a half years before you throw a leg over for the first time. Even if you have your own place that is an appropriate setting for a very young horse so you are not paying board it can still be (for me) $2-3K a year in farrier, vet, feed, hay and shavings per horse.
If you stick your money in a savings account and in a few years it’s a lot less risk to take your $20K and go buy a just started youngster. A year and a half ago I bought an Oldenburg for less than that and he’s as nice as anything out there and looks as if he will be competitive in any company, as long as I can ride up to his ability! 
(Although maybe not enough blood for the ULs–but not my goals–he’d be more valuable straight dressage or hunters anyway.)
By the same token I would encourage young professionals to set their programs up so they are purchasing the highest quality weanlings they can afford and be bringing along a string of horses with time while still riding their mature horses. When it becomes obvious that one isn’t an UL candidate you still have a very nice expensive horse to sell. When the UL ones get there you don’t have an owner that can take your horse away. There are surprisingly few professionals who do this. But the couple I know that do it will also sell less than 50% interest, so still retain majority decision making while picking up a little help on monthly expenses. But these are professionals who are set up better to take on the risks and have a broad enough skill set to bring on the youngsters from the beginning.
ETA: I should admit/clarify that I purchased a coming 4 year old just started (not yet jumping) in Feb 2020. I liked him so much I purchased his 2 year old full brother later the same year. I’ll easily have the same amount in him that I purchased his older brother for when he’s the same age. I also have a yearling I’ve bred and one (hopefully) in utero. I don’t expect to make a dime on either, but maybe I won’t lose my shirt–I’ve a friend with a stallion and this is more about helping her get nice horses on the ground to promote him. I have my own farm.
