We don't even know how powerful shanks was at the time that blackbeard gave him that scar. For all we know shanks could of been fodder at the time and blackbeard could of been stronger than him.
If this was the case he (Shanks) would not have given that scar so much importance as to tell it to whitebeard while making it clear that he was not fooling around when he got that scar. Shanks knew teach was strong. Also the fact that teach taunted shanks on the same scar, highlights the importance of that scar. So it was not a simple fodder got scar.
Characters get stronger over time. For Example: just because Sanji got his leg cracked by vergo does that mean vergo will prove a challenge for Sanji later down the road after he's gotten stronger?
Well as long as vergo remains put and does not get stronger that logic works.
..Also Shanks lost an arm trying to save Luffy from a sea king so BB giving him a scar isn't really a big deal. Hell even whitebeard received a stab to the chest by squado (how ever you spell his name).
Watch this at 3:28,
[video=youtube;coh52RaVxuM]https://www.youtube.com/watch?v=coh52RaVxuM[/video]
Shanks was pretty clear that he was not at all careless or goofing around when he got that scar and tried to highlight the importance of it.
VA's vary in strength dramatically, For all we know Bastille could of been one of the weaker VA's. Also It was implied that bartolomeo no diff'ed maynard, WB no diff'ed John Giant and rose (that's to be expected since he's WB though lol), Even Momanga was helpless and had to stab is hand just to not be affected by Hancock's power.
No matter how much the VA stregth may vary, I have never seen a VA getting punked like they were by WB in marineford, by dofy in PH and Sabo on dressrosa.
Bartolomeo is a separate case, since his DF is a unique one. The number of people who could destroy his barrier may be just a handfull, and the fact that most of them get caught unaware get their wrist broken.