The core of the issue is our Mastery combined with our current talent trees.
Our mastery stacks. So if you have 20% mastery, and you have 5 HoTs on someone, that’s 100% increased healing on that target.
That is great in theory, and it has worked out well in the past, but because of current talent and ability bloat, it’s now possible to put a LOT of HoTs on someone. We have Germination which allows you to cast two Rejuvenations on the same target. You have little mini-HoTs like Spring Blossoms (from Efflorescence), as well as the HoT from Cultivation, which don’t do much healing on their own, but add a Mastery Stack. Lifebloom can be talented to add 3 Mastery stacks instead of 1. Regrowth can be cast lightning quick now.
The result is that instead of this being a wonderful fun Mastery to play around, we got nerfed so that we now MUST have a ton of HoTs on anyone just to be able to put out decent/normal healing. For example, if you just toss a Regrowth on someone who needs an emergency heal, it will be incredibly weak. A Druid is balanced now around the idea that you will already have multiple other HoTs on that target first.
But unfortunately, you can’t always predict the future, to know who to put HoTs on in advance so that you can actually do respectable healing to them. In PvE, you have more latitude since encounters have somewhat predictable damage patterns. In 5-man groups there are few enough people you can mostly keep at least some HoTs on people basically all the time. In PvP however, things are chaotic, especially when people are also purging your HoTs.
To add salt to the wound; Grove Guardians, which were added last expansion to patch a hole in our reactive-healing toolkit, were nerfed badly so that they no longer fill that role. Instead they now mainly serve to buff our HoTs, turning them into just another spell that we have to cast ahead of time just to be able to do decent healing with our normal spells.
In PvE, if you are really familiar with your entire toolkit, know how to put it to use, and are great at getting out spells in advance, we can still do okay. In PvP it’s a disaster as you’re not going to be allowed to sit there and ramp to the degree we need when facing off against anything resembling a skilled opponent. All that remains is casting weak nerfed spells that can’t stand on their own on people who have few or no Mastery stacks.