I've been thinking for a while about taking the bike to the US but I've read sop many tales of the TSA opening boxes and things getting damaged/going missing its really putting me off.
Any possitive or negative experiences? Is it only on exit or on entry, or both ?

Dunno if this is helpful, but I lived in the US for a while and moved back to the UK in 2018. At the time I brought my bike back with me (purchased over there) as it was surprisingly cheap to do so. I acquired a large cardboard bike box from REI, part disassembled the bike and filled up the box with helmet, tools etc to meet the max weight (27kg or something) and bubble wrap in any empty spaces.
When I arrived home, I opened the box to find TSA tape and leaflets saying they'd inspected the contents, but everything was still there and undamaged, so pretty positive from my experience. I did take more valuable stuff like GPS, lights etc in my hand luggage, just in case.

Its always a risk taking a bike on a plane, maybe more of a worry how the baggage handlers will treat the box rather than how/if TSA inspect it...!

Have done it 5 times, never had any issue with TSA etc.

Famously did have my bike smashed by baggage handlers, but that was outbound, so was likely the clowns in the UK anyway.

No problems with TSA over half a dozen visits. First time freaked me out the box had been opened, but I got used to their calling card inside.
One trip, from Manchester to Seattle, the box had gotten a bit mashed, but the bike was ok. Could have been either way, as I've had mashed boxes coming into Manchester as well.

Greetz

S : )

Never had issues. Hard case box, with a TSA approved lock they can open if need be.