Most of this argument is probably mostly-pointless as the merc isn’t even released. In kandyrew’s video he tests out turtle’s deployable cover. When he melees the wall it emits green smoke. Unless this is just some stupid bug (which is possible) maybe the wall does have a defence mechanism which will make it better. Also if turtle gets more than one deployable cover like fletcher’s sticky bombs or mines, it will probably make him eons more viable and give a lot more options. Depends on size.
To shoot explosives over the wall you would have to go at a certain distance where the wall (or area right behind wall) will be at the exact location where the explosives will land (since it goes in an arc). Turtle can easily reclaim the wall to avoid incoming artillery or laser (since it’s pretty noticeable if the marking laser is right next to you). All this is made obselete by high ground (even without explosives) depending on distance.
Many indoor areas, objective points are tight enough where a deployable cover can block off enemies or at least hinder them so they will take a long enough time to go over the cover to alert the turtle.
We’re all making assumptions however. There are so many ways to go, we can’t really discuss this before turtle is even released.
I hope he gets more than one deployable cover.

