The biggest hurdle for a QNAP NAS user to move to truenas i was told was expansion. But this problem will be solved by end of this year
Then the other thing is how to do docker containers on the truenas platform they are not familiar with. I already documented in the journal to use jailmaker to do that, and how i went about it. it’s easy but it’s commandline based. compare that to something like containerstation which is all gui click install and you are done. But still, if you can follow the guides and setup/manage from commandline, it’s really worth it, because you can have a good place to deploy your dockers on and be very power efficient.
The fan issue, go bios disable smartfans. fans will crank up to max. if you don’t do that, fans might not be working at all. At least it didn’t for the TS-877.
LCD couldn’t fix it. But if someone is more technical they might be able to make a script to do so. I’ve seen it work in an older truenas version before. So it’s not impossible, just someone needs to update the fix for the new truenas version.
then how to restore your data from the qnap to the truenas. use rsync. i tested that working when using qnap HBS to truenas.
oo and you need to be able to connect a monitor and keyboard to the nas in order to be able to access the bios to install truenas. For the TS-877 you need a graphics card to do that.
Yeah i think that is about it. All the challenges facing those still on QTS and hoping to make that switch.
It’s worth it especially if you are stuck on ext4 filesystem and no upgrade path to zfs. Or maybe your model went EOL and you no longer get updates. Or you simply want a replacement os with less bloat.
Not every qnap model is suitable for this. check the truenas requirements first. I only tested 3 models which i can confirm as working fine with truenas. I have an ancient model a TS-509 pro, but i don’t dare install truenas on it 
the hardware is way too outdated. You are better off just getting a new hardware if you plan to build a new nas. It’s more or less in a decommisioned state.