Geas is a 5th level spell with a duration of 30 days. It has a casting time of 1 minute and a range of 60 feet, and has a verbal component. Basically, this means that to be affected by Geas, a creature must be willful or restrained / powerless.
At the 9th level, a wizard can cast the spell once a day and potentially have up to 30 people under his Geas at a time. A warlock could cast it twice every 62 minutes, with hundreds of players under his Geas (18 / day, assuming a full day of 8 hours and 18 minutes).
What could you do with 540 people under your Geas which is much more broken than 30? A small army ("Are you going to fight for me")? A small town ("Will you vote for me as mayor")? In both cases, the strategic charm of fewer people could achieve similar results. Thus, even if the number of people is 18 times higher, considerable effort is required to prepare each of them for Geas (assuming they are unwilling to do so), so the case of use of the latter would be very specific.
On shorter time scales, say a day or two, having 18-36 Geased, or even 10, instead of one or two, could contribute to the realization of a plan – Geas guards not to defend the king at the right time, or what. It's smart, but barely broken. Again, you can do the same thing as another launcher with more time.
Since it allows for a backup (wisdom in this case), one could just as easily have attempted to persuade each target by using diplomacy or intimidation (or even deceit), which did not Use limit per day and can potentially affect multiple targets at a time. .
In short, in most cases, to be successful, you must master it, which means that you could have killed or eliminated it in another way with less effort and better chances of success. . The target has already been defeated. In most situations, your plan has many valid paths.
In fact, since you can only do one thing to one person by casting, nothing prevents them from talking to others about Geas (Gater's one to stop them from telling others that they're not going to be able to do it. they are Geased would be stupid), risking ruining your plans. and making it largely useless for many forms of espionage or infiltration, etc.
Geas is normally such a specialized use that I doubt very much that it should be further restricted or that it deserves special attention for Warlocks. Maybe I miss something here, but apart from a campaign / a specific world / context where it might be enough to justify restrictions, I would bet that in most campaigns, no restrictions would be necessary.