Tiling work to be complained about?

Sorry, but you cannot register it as a defect in the tiling trade if a crooked wall is tiled.

Regarding the trays: I would be annoyed if everything that tips over on my tray rolls down the front and possibly crashes into the sink. Since the walls are rarely cleaned with the water jet and generally wiped afterwards during cleaning, the sloped roof at the front would be more of a defect for me. In general: If you really want to find defects, you can argue all the different options as defects with the right arguments.

I see it the same way above the sink, but in the shower it’s not acceptable at all for water to always remain standing.

Sorry, but you can’t register crooked tiles as a defect with the tiling trade.

As written, the tiler is obliged to examine the substrate beforehand. If he finds that the wall has been plastered crookedly and unevenly, he must inform the builder that he cannot tile defect-free on this substrate.
The builder then has the option to have the plaster reworked accordingly or to exempt the tiler from liability regarding this wall.

If the tiler omits this notification and just tiles anyway, he must also be liable for the uneven surface afterwards, even though the defect is not originally due to his work. I assume that the builder must at least bear the costs for straightening the substrate during the subsequent rework (dead costs), but removal and retiling are then at the tiler's expense.

Imagine the same scenario with a bricklayer who, despite lacking a foundation, bluntly places his bricks on the ground. Nobody would say, "Beautiful wall, clean work. That the foundation was forgotten by the groundworker is not your fault," would they?

Sorry, but comparing a drywall wall that is not perfectly vertical with missing foundations is nonsense. Impact of the crooked drywall wall = 0. Apart from the shelf in the shower, all of that is completely irrelevant what the second tiler is complaining about. But it's also a classic to ask a colleague, they always find something, of course. The ugly rails and corners would bother me more, but none of that is a defect.

Sorry but comparing a drywall that is not perfectly vertical with missing foundations is nonsense. Effect of the crooked drywall = 0.

Regardless of your subjective assessment, a "defect" is legally defined and is relatively extensively standardized and judicially specified across all trades. A crooked wall (outside permissible tolerances) constitutes a defect; even if it results only in a mere aesthetic problem. The example with the wall just makes it clearer because the consequences remain less abstract. If the tiler builds on it without consultation, with the result that his work is also outside permissible tolerances afterward, he cannot escape responsibility by referring to the (defective) preliminary work. You may personally think whatever you want, that's just how it is.

But this only as a general note. I did not want to say that anything is defective in this specific case. Maybe it is, but I cannot assess that.