Philly and Denver!!!! Those 2 teams have had all the injures recently!!!
I think the problem with the field turf and possible this hybrid is the base that it is set upon. That makes the surface extrememly hard and I question its ability to naturally absorb shock instead of transferring it back up the athletes leg.
If there is an issue with this GrassMaster system I doubt it has to do with the hardness of the field. I've never heard anybody complain on that score and it is supposed to be 97-98% grass, so it has a grass-growing base.
What I would question is it's resistance to "divoting" and that it might be prone to having a cleat get stuck on an awkward hit. If so, this would account for the number of knee and ankle issues we seem to accumulate. All-grass may have just enough give and or put up a divot on an odd hit to prevent cleat-sticking to avoid some of these injuries. This is just a conjecture; the kind of micro affect that might be at play would be very hard to measure.
This GrassMaster system is very popular in Europe for soccer pitches, particularly among the elite English teams...Manchester United, Arsenel, et. al. use it. However, soccer players are not often subject to high/low or twisting tackles.