I actually had the opposite experience. Way back in the day when I whitelabelled for NetSuite, they used to do fixed bid implementations, but the only way they could de-risk it was to have basically nothing in scope. The model was essentially a fixed number of hours to train you on each module/area, and then the client was on their own after that. Especially for data imports, that was awful, because it was basically like "here's how you do csv imports. Good luck!". The clients never wanted to pay for extra hours, so basically their implementations ended up going poorly because they didn't have the internal resources to do it on their end, and we didn't have flex in the scope to do anything other than what was initially agreed to.

I think fixed bid is not a great idea, sure you know how much its "going" to cost, but I've seen projects where the scope wasn't anywhere near where it needed to be for the needs of the client. I've seen several half baked NetSuite implementations where they ran out of budget with the implementer and had to be finished by another because the relationship had soured so strongly.

Is that a fixed bid problem or an "I don't understand what you want" problem? Seems like its an understanding issue. For example if you ask me to write a script and I say "It'll take me 5 hours" I'll do it for $500. If it takes me 10 hours (twice as long), and I ask for $1000 there's 2 things: 1) I highly underestimated and that is on me for not estimating the hours correctly for a fixed-bid. 2) Key word here is estimate. If you think you are going to go over you should let them know way before you eat up the hours anyways to re-evaluate. If that happens constantly, then I can see the relationship going south quickly.

I think it's more likely a case of someone selling a project to sell software for annual licensing commissions and knowing what the upper budget was they would spend on the project.

you can find project horror stories on every side, and they're almost never a result of the way you bill the project. I will continue running my business on fixed fees ¯\_(ツ)_/¯