There was a period where broken 'lockdown' combos ruled the OP roost. I didn't see a lot of 'em in actual play, though.
It wasn't ever "CR," but level+5 or so was meant to be on the table, and a single-encounter-day 'alpha-strike' could certainly reduce the difficulty of a solo encounter, as could a larger party, as could pre-(or un-applied) errata lockdowns...
There were a couple of versions of Lolth, including an 'Aspect of' that was a 25th level lurker, not a viable encounter for a 24th level party. MM3 lurkers were designed to hit hard once or twice and go down (relatively) fast. They were probably the role most like 3e monster designs in the tendency to rocket tag, or to 5e designs with their steep damage scaling.
It could feel 'too easy' if you stick to dead-on encounters and didn't have enough attrition over day, but not to the same degree, and without The corresponding class imbalances.
Aside from that, not s'much.
We have guidelines that call out a lone monster of equal or higher level as a viable encounter.