I will preface this by saying nothing against a DoT job, I would like to see one, I just can't cut out that part without losing context for the rest.
Starting with slow. How would that work? a boss takes longer to cast their actions? How is that going to affect fight balance? If a mechanic has multiple casts, does the slow down mess up debuff timings? It is just something that is going to mess up fight flow and is likely to cause more problems that it solves.
As for a debuff that lowers the bosses damage, I will use HW as an example. WAR's Storm's Path effect was a 10% damage down, PLD had a 10% strength reduction on Rage of Halone (which was in the enmity combo so it was a DPS loss for PLD as well) and DRK had [old] Delirium, which was 10% INT down. Now, Storm's Path and Delirium were both 100% uptime debuffs and, as a result, the Devs mentioned that they had to take into account these debuffs when designing raid damage, however, with the way things were setup, you would have WAR then either PLD/DRK dependant on what the main damage was from the raid (mainly DRK at the time) and, to use something else, you effectively lost some free mitigation and it went against the idea of every job being viable for every encounter. A PLD/DRK combo was going to be inferior to either of them paired with a WAR. You could make it so that all tanks could reduce damage by 10%, however, then comes the point of, well, if you have it up virtually all the time anyway, is there really a point to it? You wouldn't be mitigating anything extra as the devs would just increase the damage to compensate, so it really wouldn't be doing it's job then. Stuck something like that on a DPS, where you could have 100% uptime, and it could either be an amazing job, or terrible and it would all be down to it's damage. You could make it a cooldown so that it isn't a 100% uptime, though Feint and Addle exist.
As for reducing damage, I will make the assumption that it is for all damage (see HW piercing meta for why restricting damage types is not healthy), of which, it would have to be very very low (assuming 100% uptime). If it is more of a debuff during the burst window, that then comes down to encounters where you have to burst down groups of enemies. You can have a debuff that affects the one enemy, or a raid buff that hits everyone and benefits everyone for the whole time. I would say this is one of the contributing factors as to why the piercing/blunt/slashing debuffs were removed.
What you also have to consider is that every job has to be able to clear everything with any (sensible) party composition. With that in mind, I can see a heavy debuff job either being too strong, or too weak, which makes it either mandatory or not desired.