I too have been through BKK many times, and have not had a single incident of pilferage or loss (or anywhere else for that matter). I'll go further and say that it's unreasonable to suggest that a night-time transit in BKK would mean pilferage is inevitable.
Lost/damaged baggage can strike at any time, as can pilferage - irrespective of the location/system. granted, some places have a higher likelihood than others
It still amazes me the number of claims fo "pilfered" items including cash, watches, jewellery,
passports! etc. Common sense says don't leave anything you value in your checked baggage, that you can't keep on your person. Always buy a decent padlock and throw away the free one that came with your snazzy case. Use a sturdy strap too, not matter how "world-proof" your bags are claimed to be.
TAGS do all airport handling, and with the terminal expansion underway, it looks a bit chaotic. Don't worry though!