Depends on the tolerances of the guard and opening on the wet side to be honest. I printed one for my MP40 on the other side and the tolerances are a little loose so when the powerhead turns completely off it dislodges and falls. Just a matter of getting the tolerances perfect so it's easy to put on and off, but no loose, so when the power goes off it stays attached.
One problem is there's so much surface area that water movement around the powerhead will dislodge it in the event power goes out. In other words, the momentum of the water around the powerhead will dislodge it before settling. If the tank is perfectly still they stay in place. Again, I think this can be solved with better tolerances.
Not too big of an impact on flow; I added a few more openings on the MP40 model and it seems to do pretty well. Overall stoked and they appear (to me) to be safer than the thin mesh ones like what you get with the gyre pumps. Hopefully I never have to find out!