Well Gold Factory has a theoretical Return To Player (TRTP) of 97.0% - which is above average for online slots.
But the most important thing about slots is the variance (or volatility):
An extremely low variance game would be boring to play as you'd likely never hit any decent sized win.
But equally a very high variance game can be frustrating as it can go for AGES giving you next to nothing, but with a small chance of a monster hit.
Different players like different things. Personally I prefer low to medium variance = more play-time, but can still often be profitable.
Gold Factory is medium variance - fun to play, but very frustrating if you don't make the right picks in the bonus round.
I only have stats for a few dozen MG slots, of those, the ones which the bonus features hit frequently on are:
Dogfather (and it's clone Halloweenies): Free-spins one in 76, Pick feature 1 in 84.
What on Earth: 1 in 78
Moonshine: 1 in 88
Ho ho ho, Twister and the other clones: 1 in 94
Most of the 9-liners (like those in the post above): 1 in 143 spins.
Of course, the more frequently the features hit, the less likely they are to pay BIG - you can't have it all ways!
Good luck
KK