The short answer is that while they have amazing MARs, AD pools, and defences, they aren't really set up to capitalize on any of it.
The long answer is that they have more weaknesses than strengths in quite a few areas- in some ways, they absolutely wreck certain other fleets- Dindrenzi played by a non-Ryjak-strategy commander (camp-and-snipe as opposed to Ryjak's focused advance, then escape strategy), Terrans, and a handful of the alliance factions often eat dirt against Relth... BUT, more factions don't than do by a wide margin. And the reasons are really surprising, but can be easy to understand when compared to another fleet.
Let's start by looking at Stealth Systems- it's a great MAR, but the majority of ships that have it, say most Core 6 Destroyers, Works Raptor, and such, capitalize on it by having weapons that reach out of that range, and then lose that defence within it, where they don't really want to be anyways. Aside from the new Gunships, Relth have -nothing- that can act similarly. Aside from really expensive T1 squads (pair of Battlecruisers, Carrier w/ Cruisers, Battleship w/ Light Cruisers) torpedoes are on the weaker side of average, and very few squads throw appreciable dice in RB3, which, as primaries, are still only halfway out of the bubble. When you consider that in Patrol games, the most common fleet level played at by a wide margin, you often need to buy Stealth for your T1 because you likely won't have other T1s to reliably buddy up, you're paying 10 points for something you need to keep chip damage off in the opening turns but are actively trying to get as little use of as possible- there are only so many turns in a game, and you need to clear that gap and start hurting things seriously. Now, think about how that factors into ships that don't buy it, but already have it- how much hidden cost is that adding to each hull? It should cost -something,- because it is a bonus, but when it, again, is something your offensive loadout actively does not synergize with... You have to wonder.
Cloaks, too, have unusual properties in that regard, but it doesn't work to compare them to other cloaked ships. This time, look at Sorylians and ask yourself what they do that works. Why Sorylians? Their weapon profiles are very similar to Relth- similar linked AD, with a weak foreward fixed and far stronger broadsides in Primaries. On the approach, they throw snall AD pools, and then clobber once they're amidst the enemy. Still, they are considered one of the tricker factions to play for a variety of reasons. Why does this matter? Because everything they do similarly to the Relthozans benefits from the way their defences work, with a few notable (cough Falx cough) exceptions. With weaker AD in the front, they can still effectively chip on the approach. With Relth, where cloaks are your protection rather than a shield (And I would argue that the cloaks are better but for the following point) your already meagre foreward forepower is cut further- doubled down on by the lack of additional squadmates Sorylians benefit from. So consider that as you approach, your enemies benefit from both your cloaks and their own defenses, whatever they may be. So, while Sorylians (who also have a slight speed advantage, which is not really super notable but somebody might care) both do and take damage on the advance, their defences never turn off. Relth gamble on doing a smattering of chip damage on the advance, but are almost certainly going to take more than they deal... And then lose their primary defensive system when they actually want to fight. This is actually my main point with regards to Oroshan- they both advance rapidly AND hit hard in the direction they advance in. They're set up to get past everything you're supposed to rely on. Coincidentally, if you look at fleets on the flip side of that, like Terrans or Works Raptor, you see some of the Relth's far more balanced matchups. But remember, Sorylians also have a few long-range foreward ships, too, while with a few exceptions Relth offensive profiles are quite... Same-y accross the fleet- and even after this comparison of fleet synergies in which the Sorylians look like champs, they're still the second most difficult faction to play from the core 6, and although Kedorians, Pathogen, and STL might join them when you add in all the rest of the fleets, Relth and Sorys are still in that bottom subset.
Ugh. I have more thoughts on this, but I don't want to be making it seem like Relth are brokenly bad, because they aren't. They just have a lot of attributes that are against their favor in small ways, you know? Like, look at what happened with Heavy Cruisers, early last year was it? They went down 5 points per ship and jumped from being "too expensive to consider" to an auto-take for a good chunk of the Relthozan community at that time. Five. Goddamn. Points. Was all it took. Widows, at the same time, got a DR buff, and are now great and Nidus went from 25 to 20 points, meaning a squad with wings cost what a squad without previously did, and are now great too. It's little things that make a difference.