I too am of the opinion that against better defenses, one shouldn't play slow tactics. Slow tactics should be played against better OFFENSIVE teams, not defensive.
In slow tactics, the team will try to find a good look. If it succeeds, the shot will have 70%-80% of chance to go in (these numbers are fictional).
If the team doesn't find a look and the shot is made in last 5 seconds of the attack, the shot will have very low change to go in (for example 20%).
Playing against very good defense will result in most shots taken in last 5 seconds of the attack.
In fast tactics, most of the shots will have an average chance to go in, let's say 45-50%. So why should anyone advocate slow tactics against better defensive teams? Slow tactics should be played against teams with much better offense than your defense to keep the point difference low.