Just under half of asylum appeals succeed, which suggests to me that a significant proportion of initial decisions are wrong. The success rate used to be 29%, so either the quality of decision making has got worse or more people are aware that they have grounds for appeal.
To take an appeal to the Upper Tribunal, you have to have good grounds that the lower tribunal's decision was wrong in law, you can't just do it because you feel like it. And if you want to challenge an Upper Tribunal decision, iirc you have to get leave to have a judicial review, which again will only be granted if there is a lot of uncertainty about the UT decision.