So 0.9 added some new toys to the AI factions, both of them possessing more large mounts than any vanilla playable ship.
One of them has all the mounts locked forward on wide wings that mean it won't be able to bring its full firepower to bear on anything smaller than a broadsiding Conquest; the other has three out of five of them on turrets, and the last two very tightly packed at the front that let all guns fire on the same point.
One of them has a clunky front shield generator, the other has a high-efficiency omni.
One of them can turn rather quickly to more easily expose its backside to the rest of your fleet, the other can instantly reposition, teleporting away from danger while simultaneously changing its facing
One of these ships appears only once in the entire sector (as far as I know), the other sometimes appears multiple times in the same fleet.
The Radiant was a nice wakeup call the first time I fought it as part of that one quest, and after the dust settled there was a cool reward at the end. But then, much later, I tried going into red security systems and encountered multiple per fleet, and it became an exercise in frustration. I get that the Radiant is supposed to be challenging and that redsec is supposed to be dangerous but this is all in a really annoying out of control cheese wheel sort of way. Like, of course 5 Tac Lances and 4 Sabot Pods is a challenge, but that doesn't mean it's fun to fight. Just reinforces the already stale fighter meta because any ship that doesn't have fortress shield is going to get instantly sunk the moment it draws the Radiant's ire. Even after defeating one Radiant and its fleet, once you're back on the overworld there's probably going to be another fleet converging on your position, possibly even with more than one Radiant present. At that point whether or not I even want to bother clearing out the system all comes down to the question of "Is this fight worth it? Because I'm not actually enjoying myself", and so far the answer is no. Red beacon systems simply have nothing to offer me at the stage of the game where I would actually consider myself "ready" to visit them. I have colonies already, so I'm not exactly looking to settle on new planets. Said colonies already have synchros and pristine nanoforges installed from salvaging and exploring ruins in other systems, so I don't need any more of those. And those colonies have banked me more than ten mil, and I would like to stay out of trouble with the Hegemony, so I couldn't care less about finding more alpha cores to turn in or install. All there is left that I can think of is the prospect of a fun but challenging fight, and the Radiant just doesn't deliver on the Fun front- at least, not in the quantities at which they are encountered.
Meanwhile, the Guardian was incredibly underwhelming for something I've only ever seen in one place, and completely alone at that. The only one I've ever fought, I destroyed before I could even really ascertain its threat level, because when I took a Dominator around behind it to shoot its engines, it activated plasma jets, turned to face me, and got it's unshielded aft section blasted to pieces by the rest of my fleet. Maybe there are more domain relic-related plans for the future where this thing can show up, but I feel like right now it is painfully underutilized. I really think it should be made tougher somehow, either in the form of altering the ship's stats or simply giving it some allies so I can't just immediately cheese it in 30 seconds by drawing its attention with a single ship and letting the rest of the fleet do all the work.
tl;dr In their roles as boss-level defenders of the colony growth modifier and the quest planet, these two ships are fine. But I think the Guardian doesn't show up enough to be appreciated fully, and the Radiant is way too ubiquitous where it does. Also what's the point of red beacon systems again?