It's been shown that there is some balancing effect (many like to call the elastic effect) on the training of other skills
So for example if you imbalance a player out increasing their IS significantly, ID/RB/SB trains more quickly or if you increase HA, OD/PA/DR trains more quickly. I did this with my big C who normally would have popped OD once every 4 weeks at age 21, he ended up popping a random OD because of the imbalance.
However the same effects of cross training, variable skill training, trainer skill, age, height etc still apply so it is not as simple as take any player and you can train their skill up fast and easy (the forums aren't always the best source of information, it is always worth checking it out yourself). The reason for the one on one training early is precisely to encourage people to improve secondaries and in doing so they also give themselves some benefit from the balancing effect. I do not call it the elastic effect because there is so much fluff on the forums about it that and some of it is bordering on mythology. Training has changed over the years, a few years ago they did a training boost for a couple of seasons so rumours of the 9 pops in 10 weeks just isn't realistic for anything but for a max height or min height player with a level 7 trainer. Reading the more modern posts that do not reference 2010 are best to refer to when looking at it.
Because of so many variables, the 'training code' may never be cracked but the training simulator developed in the US is still an incredibly valuable tool to plan training, if you plan imbalances in (although don't go mad with those as cross training may freak your player without you meaning to) you can develop a very tidy player and get the joy of pops you had not expected.