You're absolutely right - I always ignored the finishing attribute for my ML / MR so far...
Using this tactic, I learned one important thing that helped me greatly boost my results, I found that "Finishing" attribute is a very important attribute for ML/MR positions in this tactic because ML/MR/STC positions score about 90% of your goals in this tactic and if your players at ML/MR positions have "Finishing" attribute less than "14" then you'll be struggling a lot because they'll be missing 90% of their scoring chances.
I suggest you'd better get players for ML/MR positions that can score and also it greatly helps if they have "Fairly Strong" rating for the weak foot.
I find that one of the main reasons why people might struggle in FM is that they don’t understand how the attributes work and how important they are.
For example, if there’s an open goal and your attacker takes a shot from 1 yard distance but he’s got only “10” Finishing and “10” Composure then probably he misses and that’s how the game work. Yes, something like that in real life would be impossible but this is a game and “numbers” determine everything here.
You said right that in this tactic ML/MR/STC positions score about 90% of all your goals and if your players at these positions can't score then you won't score and will be losing... it’s simple like that.
There are a huge amount of players for ML/MR positions in the game that have these parameters:
1) They are fast.
2) They have good dribbling.
3) They can’t score.
4) They can’t pass.
5) They have only “Weak”/”Reasonable” rating for the weak foot.
All these players are quite cheap so many mid clubs can buy them but if you get such players then it won’t make you win everything because to win everything you need players for ML/MR positions that have these parameters:
1) They are fast.
2) They have good dribbling.
3) They can pass.
4) They can score.
5) They have “Fairly Strong” / “Strong” rating for the weak foot.
Also, it's very important that your players at MR position had “Very Strong”, “Strong” or “Fairly Strong” for the left foot because he’s got “Cut Inside” PI and if he’s got “Weak”/ “Reasonable” for the left foot then he’ll be losing the ball every time when dribble.