I have run UE4 on both and I personally can’t tell the difference. I did not experience a FPS drop, hitching or any noticible performance hit after moving from SSD to HDD. I spoke to our Enigine QA department as well and they agree it should be fine either way. I would recommend HDD personally, just because of SSD’s writing limitations and that you will be altering files/assets frequently with UE4.
I hope this helps answer your question. If you do experience something different, please let us know so that we may look into what is happening.
I do not notice the difference, but I recommend installing it in the solid engine, and if you do not have more storage space you can save your projects on the hard disk.
it might only increase load times a bit. all it needs will be stored in the RAM anyways, but when loading a new asset or loading the entire project itself, it needs to load said data into faster accessible RAM or swap memory (temporarily stored data on drive)