That said, just because you had good luck doesn't mean that it's stable.
Here's the most trivial way I can think to explain this:
Check out Arch News.[0] Ctrl-F (Find) 'manual intervention'. Six years of results on the first page ; 13 instances of 'manual intervention required'.
Reliability != stability. Stability usually implies a platform on which one can use and develop for without expecting common major changes, if ever.
As for the anecdote of using the [Testing] repositories, a users experience with such things really depends on their use of new and currently developing software.
A simple computer with simple peripherals that is used to run emacs all day isn't likely to be broken by the Testing repositories.
Personally , my anecdote : the second someone starts using Arch for something new-fringe (hi-dpi, touch, tablets, SPDIF, SLI, NVRAM, new window managers, new x-windows replacements, prototype schedulers, filesystems, or kernels, PCI pass-through, exotic RAID configurations..) [Testing] repository is an act of masochism. It's only a matter of time before something drops out.
I found that my best bet was chicken sacrifice and Opus Dei-style self flagellation before each [Testing] 'pacman -Syu' . At least then I had a 50/50 chance of the next boot. But, granted, I use a lot of weird or fringe hardware.
All that said : other distributions don't have better [Testing] repositories. It's just that [Testing] is for .. testing. It's unstable by its' very nature.