Seems everyone failed, I shall step in

@Marc3l What I've noticed that players DO tend to gain more for the highlighted attributes in the new version. For example, this is one of my ST:

You can see the difference between the grey and white attributes. BTW all the positions and SA were as it is 5 seasons ago when he came as a recommendation.
This is another one from @nikolgiorgos post:

If someone could train a guy till 15SP in the old version and then auto-assign them, I could've concluded that important attributes are invisible, but present in the old version (don't have enough rests

). But since the OP is new, they don't need it.
Well, training players a new role or SA is a waste of time. Those 40-50 SAs could've been used for the raw quality. Also, a DC needs to focus on only 8 attributes, so it'll make him a master in defence. Adding a fullback or DMC position makes it probably 10-11 attributes to focus. Just for the sake of being an all-rounder, 3 position players aren't worth it.
I choose 2 position players mostly, wing players must play another position, since it becomes very limiting that they can play at only one spot. I never tend to play DML/R and ML/R, so I look for DC/DMC, DL/DC, DC/DR, AML/AMC, AMR/AMC, ST/AMC, ST/AML, ST/AMR. A single position DC or MC doesn't hurt (rarely you get double position MCs, and single position DC is a great defender since he has to focus on defence only — I have one, he's 120% in defence and rates consistently above 7). STs, I prefer dual position since they frequently have bad days and I can quickly get them back on track by swapping with another player. I prefer double position for tactical advantages, not for a more diverse skill distribution. I recommend you NEVER to have wing players with single position, they are very limiting. I once had a DL. He sucked every 3rd game, but I could do nothing but bench him. I nowadays swap players' position if they are playing bad.