It's a sign of how long I've been gone that I have no clue what PBR is and why so many people are unhappy about it.
The issue with PBR is that upon introduction it really was a mixed bag: for some people it went well and flawless, while others had all types of major problems and hiccups, me included.
PBR itself is just another way to compute the lighting, enabling a pretty accurate display of metals for example. It's a really big improvement on visuals, which can be used by content creators to do stuff like not possible before. PBR also enables us to finally have real functioning mirrors in SL, not just fake ones.
The downside of it is of course that such new stuff needs some horse power. Here lies one of the first problems Linden Lab might have underestimated: you cannot turn off the advanced light model any longer. With the rollout of PBR it is always enabled by default.
So of course PBR in action is having some impact on FPS performance. The interesting thing though according to trustworthy benchmarks is that LL managed to offload this work load mostly on the GPU of your rack, so in theory you should see no really dent on FPS, probably even some improvements. If your GPU is capable enough, that is. Of course you need also the right CPU, as always.
The issue is that this is the optimum some residents of SL indeed experienced. But Linden Lab also changed major parts about the texture download queue in a way, that suddenly rezzing avatars and surroundings became much, much slower for many residents with a PBR enabled viewer.
For example when the first iteration of Firestorm PBR enabled came out, the time required to display my own avatar on an empty sim increased from 10 seconds up to 2 minutes. When going to medium attended meeting points some textures/avatars were still not rezzed at all or partially grey after 5 minutes being there.
Another issue was that the PBR rendering in the LL viewer was better than in Firestorm, because Firestorm took an older version which had indeed a major performance hit and this really pissed off many people.
In fact Linden Lab then put this on priority, and improved PBR in their own viewer quickly as well as Firestorm. Personally rezzing times are still not on par with the last pre PBR viewer, so I refuse to do that switch because I don't care about eye candy when a basic necessity - rezzing - is still working sub par.
Also LL managed to implement PBR without giving people a helping hand about the changes they need to do to have a nice, fluffy SL experience. For example with PBR introduced the standard environment settings were really looking kind of dull. This is why now many tip videos are around with people sharing EEPs to reproduce the former SL looks again.
Another thing is that due to PBR your sim lighting might suddenly feel... off a little and you don't know why. Again LL will not explain it to you. The reason why is because due to PBR now stuff suddenly matters which didn't matter much before. For a good PBR experience these are light sources and reflection probes.
The server puts reflection probes automatically over a sim, but their location might not always be the best to match objects. Moving light sources or adding them might also be a thing depending on your build.
Also thanks to the introduction of PBR neck seams are back again.
A future annoyance by the way will be the technical switch of Voice from Vivox to WebRTC, which is undergoing right now. At least Firestorm will support WebRTC only in PBR enabled viewers, LL of course as well. So using non-PBR viewers will become more challenging over time, if you want to do that and using voice in SL is your thing.
So in short: while PBR improved visuals a lot, it also added a lot of complexity to SL at least Linden Lab is not explaining the residents... much, it meant performance drops/issues for many, also a really dull visual experience with it enabled without telling the people why.
Also some images from a resident. This is the day cycle on midday using PBR without any manual intervention.
This is midday by pressing SHIFT+CTRL+Y.
And this is "Midday (Legacy)".
Anyway this Reddit here gives really interesting insights, many of them came true.