On page 287 of the Dungeon Master's Guide, it says, under "Changing Spell Lists":
Be cautious when changing the warlock spell list. Since warlocks regain their spell slots after a short rest, they have the potential to use certain spells more times in a day than other classes do.
Would the spell Geas be an example of a spell that would be unbalanced if added to the warlock spell list?
Geas is a 5th level spell that has a duration of 30 days. It has a casting time of 1 minute and a range of 60 feet, and has a verbal component. Largely, this means that in order to be affected by Geas, a creature has to be either willing or restrained/helpless.
At 9th level, a wizard could cast the spell once per day, and potentially have up to 30 people under his Geas at a time. A Warlock could cast it twice every 62 minutes, having hundreds under his Geas (18/day, assuming a full 8 hour and 18 minute day).
What could you do with 540 people under your Geas that is significantly more broken than having 30? A small army ("You will fight for me")? A small town ("You will vote for me as mayor")? In either case, strategic charming of fewer people could achieve similar results. Thus, even though it is 18 times as many people, it takes significant effort to prepare each one for Geas (assuming they are unwilling) such that the use case for it would be highly specific.
On shorter time scales, say a day or two, having 18-36 Geased, or even 10 or so, instead of merely one or two, could help a plan come together - Geas some guards to not defend the king at the right moment, or whatnot. It's clever, but hardly broken. Again, you can do the same thing as another caster with more time.
Since it allows a save (Wisdom in this case), one could just as easily have tried to persuade each target using Diplomacy or Intimidation (or even Deception), which has no limit to uses per day and can potentially affect multiple targets at a time.
In short, in most cases, in order to successfully Geas someone, you have to subdue them, which means you could have either killed or otherwise disappeared them with less effort and a higher chance of success. The target has already been defeated, how your plan with them proceeds has a number of valid paths in most situations.
In fact, because you can only Geas a person to do one thing per casting, there is nothing preventing them from telling others about the Geas (Geasing someone to prevent them from telling others that they are Geased would be foolish), potentially ruining your plans and making it largely useless for many forms of espionage or infiltration, etc.
Geas is such a specialized use spell normally that I highly doubt that it needs to be further restricted or that it deserves special consideration for Warlocks. Maybe I am missing something here, but outside of a specific campaign/world/context where it may be OP enough to warrant restrictions, I'd wager that in most campaigns, it would not need any restriction.
