MateJ89- I personally didnt use ARC so I dont have first hand experience with that issue, but I am aware of it. Yes this is a well known issue with Onkyo across all levels of receivers. Even Integra (Onkyo's high end brand) has had this ARC issue. From what I read online about it, a common low cost fix is to replace the HDMI cable with an HDMI certified one (has a hologram HDMI logo sticker and serial number) they arent anymore expensive and amazon has them. Another cause is actually not Onkyo's fault entirely. The ARC issues seem to follow LG and Sony TV's, some models have firmware updates that solve this, but not all of them. If you have a sony or lg tv, this may be a permanent issue for you, I would consider the return and pickup another brand. I really liked the sound of the 787 when I had is, it's actually still my favorite sounding receiver but I couldn't live with the cutting in and out. I went Pioneer elite (owned by ONKYO) and I'm glad I did. I hope this helps and please fact check me, maybe something has changed since I read these things.
As far as THX certification, I dont know how big your room is, but that's all it's for. It does nothing for small to medium sized rooms. I would highly consider going Pioneer, I have had nothing but great experience.
P.s. if you do return it, look for a reciever that will be HDMI 2.1b compliant. 2.1b supports eARC which is ARC on steroids look it up it's pretty cool.