I honestly feel like it just comes down to the riptide, and by extension the riptide wing being too good against your average opponent. Or more specifically, too cost efficient. They're just not that great against the super competitive lists that tend to make it far in tournaments. Riptides could be base 250pts and people would still get them.
Any other units that might be considered 'too strong', like broadsides, MLs, etc. Are compensated for by other obvious tau weaknesses.