I will start off my thoughts.
Halberd: Out of the three stages that have been getting the axe lately in certain regions and in majors, I feel Halberd is the worst offender and has no business being considered as tournament legal. Without getting into a very in-depth thought process about it, my stance on it is simple. The hazards are not the issue, the ceiling is. As a Meta Knight main I am well accustomed to how abusive this stage can be. To illustrate exactly how significant the low ceilings on this stage are, Sheik will typically die to Meta Knight's Shuttle Loop on the ground at 109% on Battlefield with optimal DI accounted for. On Halberd's 2nd transformation, Sheik dies at 79%. This difference does not even consider rage, which normally accounts for a 25-35% difference in kill power. This is simply too much. Players will often complain about Delfino Plaza for its infamously low ceilings during transitions, but these instances present counter play. Halberd's conditions are static, and are much more similar to walk offs than traditionally low ceilings. It makes match ups volatile, skewing the risk reward on hit confirms to a degree that does not properly represent player skill, and this becomes even more exacerbated with characters that can utilize this stage's potential, such as Meta Knight and Zero Suit Samus.
Delfino Plaza: I have mixed feelings on this stage. I see the legitimacy behind the complaints players have. Looking at the data for the blast zones, 13 ft with platforms on some transformations is simply too low. However, this only happens for a short duration and presents counterplay in the instances where it can become abusive. I feel that most players who complain about this stage are largely uncomfortable playing on it because they are not cognizant of the transformation patterns, where the blast zones are low, and the important time windows that top players need to be familiar with if they want to succeed here.
Castle Siege: Within a 6 minute match, each transformation will last 45 seconds. This means that the 2nd transformation, the primary concern regarding this stages status, will be present at 5:15, 3:00, and 0:45. In most cases, this transformation will only occur twice a game, comprising 1 minute 30 seconds of the clock. While this may seem like a lot, it does not mean that 1 minutes 20 seconds will involve degenerate play. There is a lot of value provided by this stage in specific match ups that is not present with Halberd or Delfino Plaza. It is one of the only stages to have average sized ceilings with narrow blast zones, and the 1st transformations slopes, 2nd transformations pillars, and 3rd transformations tilts offer projectile protection. Other complaints accompanied with this stage include players dying during the transformations by falling off screen, but this is attributed to stage ignorance, not paying attention to the clock, and lack of awareness rather than instances where the death was unavoidable. I definitely feel this stage deserves to be legal. 7 legal stages is very small with DSR and 1 ban, and the stage list could benefit from having this available.
Wuhu Island: There have been players who feel that Wuhu Island should be considered for legal tournament play in singles. I have most commonly seen Wuhu Island legal in doubles format, but this trend has not carried on to most majors. While I still feel this stage is not legal material for singles format due to the relative stage size, if it were to ever become legal in singles, legalizing it in doubles would be a good place to start.
Lyle Cruise/Dreamland 64: Lylat Cruise is not a common favourite among Smash players due to its tendency to disrupt combos, interfere with projectiles and favor certain recoveries over others thanks to the stage tilts. However, while I think Dreamland is a more tolerable stage, Lylat should be considered as the 5th stage for tournament striking assuming full list striking is not being used. Dreamland, while different than Battlefield due to its lower ceiling, deeper floor, wider platform distribution and wind, presents significant redundancies in the stage list when playing against characters that effectively utilize the tri-platform layout. Playing against character like Zero Suit Samus in this format often affords them the luxury of choosing between Smashville, Final Destination or Town & City game 1 because the opponent is forced to ban Battlefield and Dreamland together.