It's not about "your Scarabs have to fight 1v1!!!!1!!111@," it's about how completely overboard ridiculous the Scarab-worship has gotten. The unit was fighting Wraiths--the S6 guys with Rending and a 3++ invul--y'know, likely the best CC unit in the entire Codex. Then he describes it as if the Scarabs (presumably seven of them) swooped in to the rescue. Scarabs who--even with Zahndrekh's help--should contribute a grand total of one wound to that combat.
Sure, maybe that actually happened. And maybe somewhere, ten Fire Warriors charged a unit of Paladins and won combat and broke them and chased them off the table. That does not mean you should run off to the Tau Tactica thread and start telling everyone what an amazing CC counter Fire Warriors are to Paladins, because there was virtually zero chance of that outcome actually occuring.
We're not talking about the viability of Scarabs, which is what people seem to continuously get offended over by completely missing the context of the conversation. The guy randomly talking about how his Scarabs mopped up some Bloodcrushers? He also had Wraiths--which means his army looks nothing like the army we're discussing here. You just have to say anything negative about Scarabs right now and people come up out of the woodwork going "My Scarabs scared a Dreadknight to death by looking at it!" and "My Scarabs pulled Timmy out of the well!" and so on and so forth.
Scarabs are great, when used in their proper roles--or at least when the intention to use them in their proper roles exists from the outset. They also possess some utility outside of that narrow role, like most things--the Meltagun you fire into those Orks is still going to kill an Ork, even if it's not spectacular for the job.
But the conclusion people are arriving at in their joy over Scarabs (i.e., let's make a whole army out of Scarabs!) is just incorrect. It forces the Scarabs to perform tasks they flat out cannot handle. When faced with these issues, the common response is "oh well the Spyders will handle that," which really isn't a sensical (or even really applicable) answer. You can't simultaneously go "wooo Scarabs we'll get all the charges!" and then "well, I wouldn't charge that, I'll just let the Spyders handle it" because those two lines of thought are entirely contradictory. If you're waiting on the Spyders to save the day, you've just taken away the one advantage you had, which was speed. And then the problem beyond that is that--as mentioned--there's a whole lot of stuff that couldn't care less about three Spyders, and will just stomp on them and move on. And for the most part, those things are the same things the Scarabs can't fight either.
When you're presented with something the Spyders and Scarabs can't kill, that entire army grounds to a halt. There's just nothing for them to do. They won't have nearly enough Shooting to threaten anything, let alone top-of-the-line Assault units. And it's not like we're talking about some super-rare poor matchup where some odd unit happens to counter an armybuild, we're talking about:
1). Any Grey Knights, who aren't exactly rare at the moment
2). TH/SS Terminators, likely the most common Assault unit in the entire game
3). The vast majority of Monstrous Creatures, Tyranid, Daemon, or otherwise
4). Nob Deathstars, or any other unit with multiple S6+ attackers
5). Thunderwolf Cavalry
If you build an army entirely around Scarabs, you've got no way to deal with any of that stuff, and probably a fair few other things I'm forgetting. Even if we believe all these best-case scenarios keep happening where your Scarabs fight the things they can reasonably fight and the Spyders deal with the stuff they can't, there are entire units--common ones--that neither of those two things can handle.
Honestly, let's not even discuss pure Assault armies--what about something like a Vulkan build? We'll completely ignore all the Shooting that army can put out. What do you do about the infinitely-common unit of Vulkan and his five TH/SS buddies in a Land Raider?
You can absolutely kill the Land Raider, and then the guys pile out. They then violently murder the unit of Scarabs that killed their Land Raider. And then what? You Assault them with Spyders who conveniently happen to be nearby? Okay--you cause one wound, and we'll say it actually kills a Terminator rather than being wasted on Vulkan. You then take about seven wounds from Vulkan and the four TH/SS. Two Spyders are dead, and the survivor likely dies to combat resolution.
We'll assume their positioning isn't as convenient as the Spyders, and they have nothing to Assault the following turn. So what do you do next turn? Send ten more Scarabs at them? They're still healthy enough to fully wipe ten bases of Scarabs, even if we again assume the one wound they cause goes on a Terminator rather than Vulkan.
And on and on it goes, ignoring the entire other 900 points of Marines strolling around the board. You can present a single, common unit that is just an absolute black hole to the entire concept of 30/9 Scarab/Spyder builds. Even if everything goes your way, they will just keep eating whatever you throw at them, until you've pretty much thrown your whole army away. And it could just as easily be some MCs, or some Nobz, or some Grey Knights.
It's just not a viable all-comers army in any way. It's far closer to a tailored response to mech than a legitimate tournament threat, unless you can bribe someone to make sure you play IG all day.