Shouldn't there be a saving throw involved? Or a strecngth check at least, or a dex check to catch the fall? What if there is a fixed wall behind the pushed opponent? Would he not take additional damage? I know these topics came along with thunderwave or what it is called and such, but is the to hit roll of the eldritch blast enough? Imagine if you got three blasts them, then it is almost a sure bet to hit at least once, that is a real killer then.
What I try to express is: it is not a 9nth level spell causing this and there is asumingly no save on the targeted party, no matter if it is a hill giant, titan or whatever, it is sent flying by a cantrip?
If you use the shove action (PHB p195) in place of an attack, the targets does
one contested skill test each push. A fighter can push 5 times in one round to push an enemy off a cliff.
Eldritch blast has to roll to hit contested by AC (to include shield spell and defensive abilities like Defensive Duelist) for
one check to see it they are hit and the ability takes effect.
As a rule, Proficiency in Athletics or Acrobatics is more rare than a dissent AC because everyone has to be prepared in some way to take an attack but skills can be limited to about 4/18 and many players will look for something more flavorful. A Rogue scout might have four but could easily take, stealth, slight of hand, perception, and investigation in order to scout for traps and be a dirty thief. So as a dex based class that could be good at Acrobatics tests, they have a +5 to a random 1-20 roll do defend against a fighter who if they built with this in mind likely could have athletics for up to +11 for the same test. There is also a number of ways to get advantage on athletics tests and most of the rogue abilities like Evasion and uncanny dodge will not help the rogue vs a contested skill test. The same rogue with +5 from dex adds that to a stead AC17 and could also have static defensive bonuses like +1 -3 armor, a shield (buckler) +1-3, the Defensive Duelist for a reaction of +2-6 to AC and these are all pretty common. They could have an AC15 - AC27 with a reaction to raise it to AC33... depending on their investment on defense.
My point is every enemy is different but almost every player character and NPC has some level of defense making AC stronger against these types of attacks. So before I would consider nerfing Repelling blast which while it is a cantrip, is also a
core part of the warlock class design, I would consider am I going to nerf fighters or barbarians trying to knock enemies off cliffs who have to do it in melee but have a greater chance of success? I would also consider @
Paul Farquhar's two points. 1. "Creative use of the terrain is part of the game." and 2. "Moral: don't stand near the edges of cliffs".
The implication of nerfing an ability clearly designed for this style of play and utility knowing that a player invested two core features of their class (Eldritch blast warlock spell and Eldritch invocation to add this feature) be cause of an off chance they use it strategically in a rare situation where it would actually work, gives me pause. In fact, I have played his warlock and it never game up in the campaign.. not once. Because we never fought in elevated terrain. Usually in caves, buildings, and level ground.
So I would be more inclined as GM to put NPC minions on the edge of cliffs just to encourage this style of play. To get warlocks, fighter, and barbarians throwing enemies off cliffs by any means at there disposal. I would also through in a few casters with feather fall, teleport spells, fly, and polymorph.... because not everyone thrown off a cliff is going to die. These feels like an opportunity for me to expand my skill set as a GM use skills that I might not normally use in NPCs. It might be interesting to have a cliff battle with aarakocra, Protector Aasimar, Winged Tiefling Variant, or one of the 4 sorcerer subclasses that gain flight as an innate agility (AberrantMind, DivineSoul, DraconicBloodline, and StormSorcery).
I feel like saying, "this hurts my game" is often a restriction we put on our selves that can be fixed with "unless I....!!" which results in more fun and me binging a better GM that responds to players instead of surprising them from doing what they want.