I love this concept since I always use my 3 subs in every match I attend. To put great use of this we need better control and knowledge during the match, the animation is terrible. Player ratings should be updated more often (so many times I see players with normal ratings during the match but when the match is over you noticed they performed badly) this will help us to make the right changes in time.

There is no precise answer to tankers and relegation. Even if I lose 2 of my best players if the rest of the team gets plus 1 stars it's a double win.