Functionally it ends up being similar to -Regen and -Res at the same time. For example, if an enemy has 100 health and you deal 10 damage, you've removed 10% of their HP. However, if you remove 10 HP from their max, bringing them down to 90 HP, then deal 10 damage, now you've removed 11% of their HP. It's similar with regen; if they recover 10% of their HP per tick, with undebuffed HP they heal for 10 at a time, but with a 10% debuff they heal for 9 at a time.
The real strength, however, is the fact that it's simply not the -Res or -Regen Interfaces. Interface debuffs can only stack up to 4 times on one target, no matter how many sources are applying it. The -Regen and -Res debuffs are very popular and a lot of players have them, so enemies in incarnate trials hit that 4 stack max easily. -Max HP is less popular, so by taking it you're more likely to be contributing to debuffing the enemies.