Why not sue for breach of contract / design?
We were very aware that big projects, be they software or home construction, don't ever go perfectly, so we made a conscious effort to stay calm and solution-oriented through the entire multi-year process. We came in very slightly under budget, and things that need to get addressed are getting addressed, whereas a lawsuit would have turned things adversarial very quickly.
Most large projects have some central features/processes that are by their very nature both very expensive and very complicated.
The entire prefab industry is built around minimising those things - but then something ELSE becomes the most expensive/complicated thing. You then have a situation where if you're seen as a bottleneck, holding up the expensive/complicated thing, then the total cost of the project will balloon.
Many prefab companies know this and are happy to sell modules with little QA work done, because it's unlikely that there will be serious repercussions. Once you accept the work then it'll be too hard for your legal team to fight for (or at least, not financially worth it).
I'm not saying that prefab companies are ethically bankrupt, but it IS an environment that promotes this method of doing business. I've been in construction for mining/oil&gas companies, on both sides of this situation, quite a few times. The amount of shoddy work you're willing to accept is quite terrifying once the cost for re-work gets into nine and ten digits.
Doubly so if it came after all the stress of custom-building a home. My blood pressure is worth more than that.
Oh and the 15k are only for legal costs. Don't forget rent for 3 years on top of that. And all the money that you'll still have to pay for the construction. And stress (medical bills are not cheap).
The one I remember had to do w/ $1900 worth of lost wedding photos cuz a server died or got deleted. Cost more to twist the knife in court than to just settle.