Well, of course Scarif was a major victory. What's at issue is the idea that it's the first major victory, that nothing we've seen before is big enough to cross the line into "major." The question is, how big does something have to be before it qualifies as "major?" And answering "as big as Scarif" is circular logic.
I guess I'm approaching it from the standpoint of the question the makers of the show would have to face. That "first major victory" line poses a creative obstacle for them, because of course they want their characters to have successes, but remaining consistent with that line would require limiting those successes. So the question is, where do you draw the line? Do you decide going in that your heroes will never achieve victories above a certain low level, or do you just go ahead and start blowing up Star Destroyers and handwave it away by moving the goalposts of "major" after the fact?
On the other hand, there's something to be said for a more pessimistic approach where the protagonists' victories are always limited and outnumbered by their failures. Gilligan and co. never got off the island, Mulder never proved the aliens were real, etc. We have seen the Rebels endure some major defeats, like being driven from Lothal and losing Ahsoka to Vader. Kanan even lost his sight. And with Thrawn as the main villain, it seems like they may be setting the Rebels up to suffer another major loss at the end of this season. And many of their victories have been limited, like how they lost the hard evidence of the Empire poisoning Geonosis and had to settle for mere photos. So maybe they are trying to limit the characters' victories as a matter of course. Which makes it seem like overkill that Zeb was able to take out a whole Imperial warship in this week's episode.
Maybe having a list would help. I always like lists. But I'm not immersed enough in this show to be able to compile one myself. Can someone offer an accounting of just what victories the Rebellion has achieved over the course of Rebels so far? I can think of a few, like obtaining various kinds of ship for the fleet (including the Hammerheads that proved so useful at Scarif) and destroying various Imperial ships, but I don't remember them all.