Any SMN that has done any of the larger FATEs can confirm that this is a genuine issue. Once 30 effects are stacked on the target any further debuffs or DOTs will give the 'no effect' message unless you already have a debuff 'slot' claimed that you can overwrite. I can cast all my DOTs in sequence and follow up with a Fester for 0 damage because every single one of them bounced off the 30 debuff limit.
It is a genuine issue and if nothing is done about it before CT we are going to see all debuff-heavy jobs excluded from content.