I'm late to the party but I tend to agree with USTik on alll points to some degree or other.
It's clearly intended to be a fairly sporty model and one which would be flown upside down much of the time. As such it should not have a thick lifting airfoil. But the pictures seem to indicate a fairly simple and thin airfoil. So I think it's OK in that regard. Although at lower speeds the wing may be seeing various near stall things going on which causes some yaw wander and oscillation.
Where it should be more obvious is when it's in medium cruise speed turns. If the tail tends to hang a bit low in the turns that's a great sign of too low a vertical tail volume coefficient. Namely the fin area x the tail length in relation to the wing area and span is too low so their is some yaw stability issue.
There's information out there for equations for calculating the vertical coefficient and even charts showing a reasonable range to stay within and using real plane examples of good values. I'll bet that this one is too far to the low side of things. I'd include links to these things but for some reason half my web sites are down at the moment. Not sure if it's some big web thing or if there's big storms causing technical issues.