Actually, I've found JUAI to be very accurate and I've also found that it tends to overestimate UAIs rather than producing artificially high UAIs (although that changes once the estimated aggregate hits ~465...it spits out a UAI of 100 when in fact you may have got around 99.8 or 99.85)
And as miss_gtr said, it's not a tool to predict UAIs...it wasn't designed for that.