Balanced training is good because it allows you to make changes to accommodate surprises.  Example: you have a striker with 10 finishing and 10 heading. He has 2 seasons of bloom left. You want to divide training 50% shooting and 50% headers.  You train headers for a season then shooting for a season.

Result: you cannot increase the shooting training time to more than 50% if needed. By the time you see finishing isn’t going up as quickly as expected (it can happen because of the 75% random factor in training) it's too late to do anything.

On the other hand, if you train 2 weeks headers then 2 weeks finishing for the two seasons, it’s much easier for you to change your mix to emphasize more one of the skills.

In addition, training to level 15 is much faster than training after 15. Since skills like heading and finishing have almost the same importance to forwards, it’s a waste to train above 15 in one main skill when you don’t even know yet that you could bring the other one to 15. You may end up with 16 finishing and 12 heading when you could have 15 and 14 instead, as after 15, the training penalty is 50%.

An extremely important tip: when developing forwards, train heading before finishing (but not up to 20, keep it balanced to a degree like I discussed before).

Between 16 and 18 you get a lot of physical training even if you don’t want it, it’s automatic and in ADDITION to the physical training you will get as part of the 75% random part of training. If you don’t train heading then, there's a good chance by age 19 the player physical talent will be exhausted = no good heading = dead striker.

I've made many good strikers, and I’ve seen scores of useless ones without heading, because of that mistake a lot of people make. Some great managers do it as well.

If training a winger, you may choose to train pace first, for the same reason I train heading first for forwards. For a forward I wouldn’t train pace first unless heading is already quite high naturally or if he was pulled with poor pace - because he will get alot of pace training automatically.