How does the shield around Wakanda work?

The shield seems to work against blocking initial impacts against it but can't cope against a sustained force and it begins to break down. Note even against the Outriders it blocks their initial attack into it but as they keep pushing and forcing against it they manage to get through.

Check out the below image and the noted points.

1. An Outrider is pressing against the barrier and is starting to break it down.
2. An Outrider has been pressing against the barrier for a while and starts to push itself through
3. An Outrider has just hit the barrier; the pulse ripples along the barrier but it doesn't break.

So, whilst the barrier can block the initial energy thrown against it, it does no good against a sustained force that the Outriders are able to apply to it.