it would be nice to have comments for every chess puzzle where users can discuss what they think about. In particular, i don't understand why some problems are underestimated.
Next, it would be nice to have 'real' feedback for puzzle. For this moment negative feedback doesn't work and minimal possible value is 0.
example of underestimated problem is
lichess.org/training/104805There are quite a few puzzles, and as far as I know they are still being actively generated from computer analyzed games. It's possible that puzzle has 0 "likes" because it is newer. Also some (most? many?) players don't rate puzzles at all, they just do them.
If, by underestimated, you mean the rating is underestimated, the puzzles work with a similar rating system to normal games. Each puzzle has a rating. When you do a puzzle, it's your rating vs the puzzle's rating. If you get it wrong, the puzzle wins and is higher rated. If you correctly solve the puzzle, it loses rating and you gain rating.
"it would be nice to have comments for every chess puzzle where users can discuss what they think about"
Some people write comments in chat of game from which puzzle is taken.
example:
lichess.org/training/63486