There is only one reason why some Orthodox may affirm the non-validity of Catholic sacraments, and that is because we are not in communion. This goes back to a pretty old divergence in teaching regarding the operation of sacraments. St. Cyprian of Carthage’s sacramental theology entailed sacraments being only through the conduit of the Church, and so anyone not in communion with the Church could not have valid sacraments.
St. Augustine developed a different idea where he made a categorical distinction between validity and licity/regularity/legitimacy. So as long as certain things are fulfilled, a schismatic/heretic can have valid sacraments (albeit illicitly).
Now the West at least in the modern day has wholesale accepted the Augustinian view of sacraments over St. Cyprian, hence why Catholics today affirm with validity of Eastern Orthodox/Oriental Orthodox sacraments with little debate.
Although the Augustian view has at least been more popular in Russia centuries past, generally Orthodox hold to a more Cyprianic view. But nevertheless, this is an area of theology that hasn’t been definitively clarified binding on the entire Church, hence why some Orthodox lean more Augustinian or Cyprianic with room for disagreement. Also, neither is there any pressing need to clarify which is correct, since there’s very rare cases an Orthodox would possibly need to ever receive sacraments outside our communion.
So if you want to argue against Orthodox why Catholic sacraments are valid, you would need to show why the Augustianian view of sacramental theology is correct over the Cyprianic view.
And the indelible mark of Holy Orders is a whole nother discussion.
