That's the issue here. To beat a fleet, you need a fleet or high-level titan (or sometimes a maxed SB will suffice). The VR have bar-none the most powerful late-game fleet in the game. While others have to concern themselves with petty things like frigates, the Kultorask can just eat them and keep them disabled half the time. If the VR get's bored, it can just nuke every subtitan with Dissever (because you can bet this thing'll hit level 6 unless it's opponent ragequit before then).
Basically, your defenses against a powerful fleet are going to ordinarily consist of 4 things: frigate fleet, capitals, starbase, and titan. The problem is that the Kultorask can make itself invulnerable and completely make a chunk of your fleet worthless. Without your frigates, the VR's frigates are going to give the VR player far more DPS. With this advantage for the VR, they can proceed to steamroll you if they have an Orkulus Rex with them. Unlike other starbases which probably only get to fire at 4-5 enemies at once, Debris Vortex combined with the fact that the Orky can hit more targets at will for more DPS than any other unit in the game: more than 300 DPS on each of 16 targets. As a result, an Orkulus Rex can demolish your remaining capitals, in particular because a lot of it's DPS is in the form of PM's which just happens to be the specialty weapon of the VR.
Now that your capitals and frigates are dead, all you have left really are your titan and starbase. While the Orky probably lost it's shields destroying your capitals, it has Debris Vortex, remember? By now, all your frigates are dead and all that is just food for the Orkulus Rex, meaning that it too is basically immortal and you are now facing two invincible titans and a full fleet without yours. The next step for the VR is to press towards your starbase and any repair platforms. Disintegrators will make quick work of them both.
This leaves only your titan if you're lucky and a few stragglers. Honestly, the focus fire from the fleet/Orky/bombers will probably kill the titan within seconds and Ruiner mines will prevent the fleeing ships from leaving. In other words, you just lost your entire military force.
Orkies threatened to be OP on the attack in Entrenchment and Trinity because getting one built in your gravity well meant putting out thousands of points of damage while the rest of your fleet gets wailed on by the attacking fleet or lose the gravity well. Now you have to put out tends of thousands of points of damage against something that is sending out 300+ DPS to each target, about a third of which is PM's.
Maxed Orkies have always been the most powerful starbase. They were the first titan. Once maxed and escorted by a fleet, they would refuse to die. The reason that facing Orkulus Rex+Kultorask+fleet is OP is because there's nothing that can really do anything to stop it.
Send frigates? Nano Leech
Send capitals? Nano Leech+Orky
Hide by starbase? Orky goes to town
Send titan? Orky+bombers
The Orky fundamentally isn't more powerful than the Kultorask, but when used together and escorted, pretty much everything is doomed.