I spent 7 months trying to build a json api using yesod with a friend. We had nothing but headaches. Its insanely hard to find example code or search issues when using yesod. Hardly anything exists about it on stack overflow already so any time I had an issue I had to post it on SO and wait a day for someone to answer it which meant I could only do about an hour of programming a day. I ended up giving up used RoR and replicated more than the 7 months worth of work in a few weeks.
There is a very good reason RoR is far more popular than haskell web apps. Its just so easy to get started with rails, there is a near infinite amount of information online.
I'm not sure what else to tell you. I run three web businesses on Yesod, and it accounts for 100% of my income.
I'm also not a great programmer, and have a long history of just "hacking things out".
The stuff works, and it's ready to go, today.