Tbh the performance of Burn in Hell at the concert I attended was very well received. I think it was a very anticipated song. Of course, when you play a song like Night Crawler or Breaking the Law, it's hard for a deep cut to compete with them. But sometimes a rarity may be the dark horse of the setlist. Or at least an important core of it. I mean, I've kinda experienced situations like that this year, during the Faithkeepers gigs I was at. Panic Attack, the newest song they're playing now, got an amazing reception. Much better than Hell Bent for Leather, The Ripper or Metal Gods, for example. I remember talking to a young dude who never saw Priest before the current tour. His fav moment of the concert was Lightning Strike. I don't think they're deep cuts, they're too young for that, but they clearly aren't the crucial Priest tunes.
KK's Priest's situation is quite interesting. Ripper isn't the original singer. He's known for being a replacement. KK is the original guitarist, but at the same time, he's not THE guitarist (simply because Priest's music is based on 2 guitars and over the time Glenn became the more recognizable musician). Also, I think - because of what I've just stated - that KK's Priest plays for a more "sophisticated" audience. If you know who Downing and Ripper are, you're likely to be familiar with a big chunk of Priest's catalogue. So while playing the hits is justified and expected, they do have the luxury of playing more than just them. Especially since Tim is a very talented vocalist with very few limitations. Blaze won't sing stuff like Infinite Dreams or Alexander the Great because he's simply not able to perform them well, but Ripper could easily sing most of Rob's songs.
You're right about Udo, the decision was made due to suggestions coming from the people in the industry. However, in his defence, his tour are much more than just "the best of Accept" type of thing. I mean, lots of Balls to the Walls songs aren't hits. Stuff like Head Over Heels, Turn Me On, Losing More Than You've Ever Heard aren't the songs you hear at most Accept shows. He even decided to play Breaking Up Again, which is far from an obvious pick. Only the first Accept-era tour was focused on the basic stuff. In 2017/18 he was playing a deep cut setlist that included tracks like Russian Roulette, The Beast Inside, X-T-C, Another Second to Be... He made sure to make it interesting and not repetitive. He could have just said "Fine, let's play Princess of the Dawn, Metal Heart and Breaker and everyone will be happy", but that's not what happened. I respect him for his attitude.