Of course opinions vary, and I'm just giving you my take on things. As you see, others will see things differently, and I must say I started out the same way. I bought a couple of Star potential players for scraps, and sold them a season later for 50-100k. That's a fine way to get to learn how to train properly on a tiny investment, and even earn some money while you're at it.
The problem I had with that, is that after 2 season I was already looking at promotion, and I was just acquiring new trainees. I then had to play my 18-year olds during league games I had to win, and that wasn't easy to combine. Now if I would have had trainees with higher potential, they'd be able to contribute at that point so I could still compete at a high level and train my guys. Even if I had kept the Star potential players, they'd soon be capped out and then I'd need new trainees in a competitive environment. Even though Star potential players can be trained into well rounded, salary efficient role players, I think after a couple of seasons you'll wish you'd be training a guy who will become your franchise player instead of a great backup.
So even though Antuma has bad secondaries, you can train him for a long time, and with his solid primaries he'll be able to contribute to your team in league matches soon enough. He can be your trainee for important matches you need to win. Also, for bigs, it's pretty easy to get decent secondary skills because you can train 1v1 and Jumpshot for forwards, meaning you can train secondaries from the PF position, which is natural for your bigs so they don't underperformance because of out of position training. That way, you'll have 3 of the 6 secondaries trained easily. Furthermore, these (DR, HA, JS) are skill that train very fast, so it won't take long for him to have decent secondaries. Of course, it's not perfect if he still has bad OD, PA and JR, but at least he's not a bone headed salary monster.
Furman is a great trainee too and I must say at 2nd look I think he's equal or maybe better, at least not much worse than Antuma for training purposes. You can train him for a while too, but I think after 2 seasons you'll be itching to buy a great trainee (superstar potential or better with good starting skills), and then Leal will be the first to go, before he's become a good rotation player.
But as I said, this is just my view, and maybe you and others feel differently. I'm just sharing my experience and you can take it for what it is :) Good luck!