as far as I can tell, zones only is great for pickups and generally getting practice in regarding fights. because the objective is so simple (it's just a matter of having mid under your control) all your focus goes towards consistently winning fights to maintain that control.
i'm probably just repeating similar things a lot of other people have said but i've been trying to put some time into understanding map pools recently, what players look for and what is considered to be a balanced, worthwhile competitive experience in a tournament and i've learnt a lot from watching and commentating teams and talking to top players and other TOs to come to how i currently think about this topic.
there's a drawback to zones as there's a lot of depth in other modes of the game, such as tc highlighting certain areas of the map, the speed you may have to be able to react as a team when defending in rm, and being able to coordinate things outside of just fighting in clams. each of those other modes gives various other disadvantages and advantages to either team.
regarding disadvantages in zones it comes down purely to map geometry and i think maps in splatoon 3 on average are just... kinda average? robo zones is probably the best map mode in the entire game because of the options available regarding map geometry. but something like scorch zones is not only stupid to cap but stupid to gain more control over because taking up more space on the map means... walking across grates. getting punished there is massively unbalanced and not really anything a team should have to worry about in a well-balanced map pool designed to test a range of skills fairly. but then scorch rainmaker is better because there are more, safer options to push and quickly retake, with pushing to the final KO being extremely risky due to the advantageous positions the defending team has. Map geometry is just better in other modes sometimes and zones only doesn't necessarily allow for that balance or variety of gameplay options and skill expression. what exactly is the skill being expressed in walking across grates in single file on scorch zones when you can have... other options in a more varied map pool
if youre playing with a goal of winning various tournaments, youre always going to have to consider all modes. but zones in particular is great if you want to focus on what it's good for and what it tests in a team or group of players in a pickup.
my biggest thought on this:
zones-only tests specific skills (ie. team fights where the only advantage or disadvantage is the map terrain that zones maps offer) and at least partially ignores particular skills and possibilities to test them in a fair and balanced way that other modes offer.