Except that after Dany takes over she gets petitioned by former slaves to sell themselves back into slavery to their former masters. This implies that their former life wasn't as bad as you perhaps thought it was. You could make the argument that maybe these were higher end slaves - but it still implies that there were a lot of slaves that were treated fairly by the Masters.

Right but Tyrion was speaking far more broadly than that, based on his very limited experiences. However the fact that the slaves revolt and retaliate every time they get the chance shows that the vast, vast, vast majority do not have good lives under their masters.

You have men like Roose Bolton who under Stark rule still practiced the right to first night and killed and flayed men (this was a well known rumour) for superficial reasons. You also have Tywin Lannister who ordered a woman gang raped by an entire squad of his guards, and also sent a raiding party to burn down entire villages and rape and slaughter. Both are still well respected (and feared) Lords in Westeros. The cruelty in the east and west are equivalent to a degree; just because you have brutal Lords in Westeros does that mean Dany should come and flay/rape/slaughter the noble class?

I actually do happen to think that Westeros would benefit a lot if the Noble class were either wiped out or brought to heel by a stronger central authority, but that's a discussion for another time.