It's really not at least not in Beta 2. I got frustrated the first time I played against them. Getting outmassed is annoying but it was a macro problem on my part. I've played both races a fair bit now and I do have to say the Advent have it worse than the TEC. This is mainly due to them having multiple options to actually defend against the Corvettes. Aside from flak I have found the Flak upgrade on hangers to be highly effective when defending building against small swarms, and alot of them against larger swarms works the same way. The Advent however only have their defense vessels.
Now as I said the Corvette problem is not as bad as it seems. But it is still a problem, they are not this horrendously overpowered unit that needs to be nerfed into the ground by putting them at tier 7 and upping all their cost by a ridiculous amount. They are actually a very easy fix to them. And I have offered multiple solutions before.
The First Solution is increase there Fleet Supply increasing this by 2 cuts the numbers that someone would originally have in half. They next solution is to nerf their movement speed, they are to fast that their counter cannot catch them. If they are harassing a system unless you are TEC(im going to assume Vasari static defense is the same as Trinity). You will be unable to defend against them. They are causing the same problem as mutalisk did in starcraft 2 only its worse. The TEC have flak on their Hangers. The Vasari and Advent have no counter to Corvette spam in their static defense.
To actually fix that problem just make Phasic Trap effect Corvettes. And give something to the Advent before game release(seriously their defense tree is so empty).
Another solution I thought of along time ago and others have thought of is make the battleship have some flak on them. This will help out the Kol and Radiance in becoming viable. And perhaps even make Flak Burst(The Kol Ability), Telekinetic Push, and Jam Weapons work on Corvettes.
These are not a direct solution to them but it does help. And it doesn't require you to nerf Corvettes heavily either. They were introduced to this game and people are not used to them as they have changed the Metagame from LRF spam to Corvette Spam.
p.s: I'd also like for maybe anyone who post to add a subnote and say if you agree with this or not, but does late game tech need to have there prices reduced. I feel like this would speed up the game by a good margin.
Edit: This was posted in multiplayer and not rebellion