“Clients from hell” are real. Unreasonable, unpleasant, and, worst of all, reluctant to pay. If you think those are exaggerations, take a peek here https://clientsfromhell.net.
For complicated work that the client knows little about, hourly billing is a stairway of good intentions leading straight to the place of brimstone and ALL CAPS emails.
A client who is out of their depth will also think there is not much to know about your craft, so they see all of their wishes as “simple”, meaning “quick”, which in hourly billing equates to “affordable”. Then, experts, needing to keep clients try to do so by agreeing to price work as a “best-case scenario”, to stay as “competitive” as possible. And so it begins…
When reality manifests itself in a high bill the irate clients stick to their original fantasy, thinking you must have tricked them. If you ever received a nightmarish bill from a mechanic or vet, you know how it feels to be on the client’s side of that interaction.
Not all shockingly high “totals” end up summoning a “client from hell”, but your project is probably not the first time this happened to them. Couple that with stress and a fragile ego… and you start to smell the brimstone.
If you can knock some sense into their expectations at the very beginning of a relationship, you may avoid the need to reach for a cross and holy water when the bill comes due.