I’ve saved this post to follow what the community will come with this topic
I’m kinda surprised that there is no answer yet..
So, I will try my best to provide as much context and experience I can
For a little introduction, I’m an engineer specialised in complex system (from HVAC to Electricity and services), I’ve work on dozen of projects at different stages (designer, engineer, project management, construction management, maintenance manager…)
Now I’m involved in software architecture around building operating systems
So, HVAC is a vast field… In term of “softwares” you can found “general” ones like Autodesk Insight that is not related to public policies and specialized ones like, in France, Izuba Pleïades or Graitec ArchiWizard that are dedicated to standard calculus and simulations (local regulations like RE2020, standards like the EN 12831)
So, basically, it’s a nightmare in term of interoperability and homogeneity from the engineering part
French approved softwares relies on dozens of databases related to materials, equipments, air quality and so on
Imagine a junction of two walls, this junction is treated as an object : a linear heat sink,
A fixture for an outside heat insulator is a punctual heat sink,
A wall (or any area) is considered as an area heat sink…
That’s for the heat sink evaluation
Then you have appliances and systems,
Again : materials, regulations, treatments…
There are some works around interoperability for those elements : gbXML, BRICKS ontology, BEM (Building Energy Modeling) products
They all have some benefits… but at the end of the day it’s still a nightmare
Graitec even choosed to convert ArchiWizard into a Revit plugin to be more integrated in a BIM workflow (I think that’s because it was easier to work with Autodesk’s SDK than trying to comply with the IFC…)
• What is the gorilla?
I don’t know any. Revit is great for air conditionning systems and to draw piping networks, ultra specialized tools allow to do the sizing and some steps still need some old and dirty spreadsheets..
Some company do their best for HVAC drawings but it still ends some how in a Revit plugin that just draw things a bit faster
• What’s the niche MEP software you love (or love to hate)?
Pleïade is really awesome for whay it can cover (air quality, heat needs, insulation, life cycle assesment and others) but it’s far from being intuitive.
Insight is more like a “one click result” but here we can not certify and use its results.
• What kinds of data matter most in MEP workflows?
Well… it depends 
MEP is :
- Heating and cooling area
- Providing fresh and hot watter to places
- Renewing the air so the building remains breathable
- Providing Gas to equipments that needs it
- Removing waste water
- Removing smoke from burners
- Extracting smoke during a fire
- Puting of a fire with gas like argon, water, foam…
- Managing special fluids for an hospitals or an industrial process
- Managing thermodynamics networks with all its specifications
And that’s for the M and the P of MEP…
Now you have Electricity. Quick overview :
- calculation softwares we use : ElecCalc, Caneco, Etap
- drawing : schemeElec, SEE Electrical, Eplan
- lightning : Dialux, Relux
- construction : mostly Revit but Autocad is still heavily used
Oh I forgot, these are for electrical power distribution systems,
Then you have network (IT) systems (sound, images, voice, data, IoT, IIoT, SCADA…
• Where does interoperability fall short?
Everywhere, we might need to be more specific here
• And what would a great MEP workflow—even a small one—look like inside Speckle?
This is a really goot question but a better one would be : what do you envision Speckle to be in the next 5 years ?
From my point of view, if speckle can reliably manage all kind of engineering informations it will be a dig weight off our shoulders, allowing us to focus on building’s live data and services, leveraging Speckle as the “technical and 3D database” associated as what we could call a digital twin (even if this term is controversary, at least to me)