It has been my experience that single tough creatures are harder on fighters than on casters. What specific monsters are you thinking of?
I don't have a specific creature that I'm thinking of. Most recently, I'm reminded of a number of fights out of a Paizo Adventure Path though, where it was a single or very limited number of enemies, which were quite tough. It's distinctly possible that those were built explicitly as a "screw you" to casters; I don't know as I didn't bother going back to look at how they were built.
Single tough creatures certainly can be tougher on non-casters; some of those freaking grapple monsters that swallow characters spring to mind.
I do think that the issue of caster/non-caster disparity is more complicated than simply player competence and the nature of the power curve built into casters and non-casters at the system level. Monsters built as "screw you" to either casters _or_ melee, spells from either core, supplements, or 3rd party sources, source of a class... lots of stuff. Like I mentioned before, I've never seen a GM that allowed Tome of Battle; but they certainly allowed plenty of material from _other_ WotC books.
One thing that occurs to me in reading some of the anecdotes about how casters don't dominate... it can be sorta "swingy". Like, if you don't have a cleric in the group, Undead have the potential to really rip the party a new one; if a cleric is there, it's smooth sailing. Casters wind up being one of those things were they seem to either rock everything, or they get brutally smeared; there's not a whole lot of inbetween.
Non-casters tend to have a more mellow bit; where they get screwed is if you're talking something like Ability Damage/Ability drain, which can yank them down to nothing in a couple of rounds, or something like a "Swallow whole" attack.
I'm not sure if I'm quite explaining myself well or not...
Non-casters tend to be reliant on the whole Hit Point/Damage per Round thing. A fighter is going to do between X and Y damage per round, assuming they're capable of actually hitting the target. It's a fairly "predictable" sort of thing that can be kinda grindy.
Casters on the other hand bypass this. Sure, they've got nuke spells, but there's plenty of discussions kicking around on how going the DD (direct damage) route with a caster is the... inefficient I guess we'll call it... route.
As a side example, I've several friends who've played Star Wars Saga Edition; they universally agreed that the quick way to domination in the game was to go for Condition Track damage. It's another example of a design that's bypassing the usual armor/Hit Point mechanic and potentially short-circuiting fights.
At low levels, monsters tend to be bags of hit points relying on the Armor/Hit Point thing. As the levels increase, monsters start getting funky powers etc, as well as hit points and armor. Casters get the funky powers going along with them. Non-casters generally don't.
I think some of the problems GMs have with the caster/non-caster is related to whether or not they're deliberately trying to account for caster/non-caster divide. Explicitly looking to make things more difficult for the casters, in order to help tone them down.
It's not that it's impossible for non-casters to rock, at least if you listen to some of the stories (usually featuring some horrific Barbarian build), but the ease with which non-casters can contribute and that casters can be short-circuited.
Hmmm.... I'm not sure if I've gone off-topic here or not. Basically, my experience has been that non-casters tend to dominate and as a GM I've taken very explicit steps to curb it. I think other GMs don't always have problems because they've achieved a system mastery on par with players that make heavy use of casters. Those GMs that haven't are more likely to have issues and this gets further complicated by the play styles/goals of the group, as well as the sources being used for casters, non-casters, and the toys that each gets.
Oh and there's also the "don't be a jerk" thing. Some players try to play nice and ensure that their character doesn't dominate while others don't. The issue of character domination doesn't have to be strictly one PC outshining all the other PCs, it can also be one PC riding roughshod over a GM as well. *sigh* So.... yeah. Complicated to answer the issue really.