Long answer:
Form is unbelievably overpowered and makes an immense amount of difference. It affects dribbling, ball control, shooting accuracy, reactions, balance and maybe a few more stats. And the most annoying thing is that simple things, like completing a few short passes or doing a few tackles will boost form unnecessarily high.
If you go to the last tab from your CM and check team/player stats, you can see the form of almost each player of each team in the league you're playing. You can see that the vast majority are between 6.0 and 7.0. If you play a bit decent, even if you lose consecutive games, your players will still average higher than that. Moreover, once you got good or excellent form, you will have to be absolutely dreadful and in consecutive games to have bad form.
This means that players like Diego Costa will be easily stopped by a defender with 7.5-8.0 form, for example, if Costa has something like 6.2. And while form is a way to add realism and complexity, it makes no sense for all world class players to struggle so much to get good or excellent form.
Even more weird is that if you Sim 2-3 matches in a row, your team will drop in form to the same level as most teams in the league, even if all your team were at excellent form! Especially if you lose and draw a few games through Sim, that's it, your form is done.
My most recent example, I began with a 73 rated team in the Premier League. After 5 games, I was 16th (perfectly realistic). I managed to win two games, and all of a sudden, a few players got a massive boost in form. I went on to win 10 games in a row, which got me top of the league... Stupid.
If you are attacking and one of the Full Backs joins the attack, they may drift towards the centre if you lose the ball and you need to defend. Once they have drifted a bit towards the centre, the other Full Back may think "Oh, he's coming this side, I better go and cover his". As someone else said, they will stay like this until there is a free kick or something.
While I agree he will need to find a way to increase difficulty, from one point on, it just doesn't feel right. It's not like we can change the CPU to manual passing. It's the choice between:
our passing being child like/CPU passing being perfect
OR
the CPU making too many errors, making it easy for us to steal the ball and hard for it to create chances and score goals
Even with tactical defending, there is no "manual defending/tackling" options (i.e. the equivalent of making attacking manual)
I have struggled a lot with this. Every year I get excited and every year I feel the exact same disappointment. Either the game will be too easy or it will be hard enough to make it a challenge but with very boring gameplay that feels unfair.