Not counting the US, the only time I've had issues with a border force is at LHR.
I've had delays at AMM, PVG and PEK, due to visa on arrival/transit without visa paperwork, but I've never had the issues I've gotten at BOS, JFK and LHR anywhere I might have expected.
During my last trip, I entered the UK for 4 hours to put a checked bag into storage and then head to my next flight. Returning to collect the bag 24 hours later (after a LHR-DXB-LHR turn to use up all the sectors allowed on the ticket and rack up points), I end up getting questioned for around 10 minutes at the primary line before the agent suggests I have a forged passport and takes me to a roped off holding area while she does some "investigations".
After sitting there for a while, 2 other agents arrive and take me to the customs area for a full search of my carry on baggage, including my laptop. They found nothing that interested them, take a copy of my itinerary (which the primary agent never asked about) and bring me back to the holding area.
Another 10 mins or so later, the original agent comes back with my passport and paperwork and lets me go after claiming I was suspicious because no one would go to the UK for only ~5 hours twice with only a 3 hours stop at DXB in between.
Idiot. I've done just that 5 times in the last 5 years and I'm quite sure many others have done similar.