What are the biggest challenges you’ve faced either trying to hire developers that know ObjectScript, or trying to train developers on ObjectScript?

@Michael Breen - is a 10 on the scale of 1-10 "super crazy hard" or "super crazy easy"?

 In past (before2016),
I had 2 times the challenge to run a private training within a window of 2 years

• Are developers generally willing or resistant to the idea of learning ObjectScript? Mostly rather resistant, but forced to learn it to get the job,  
• Why? ObjectScript is not the mainstream. And in my geographic environment (middle Europe) demand is rather low. So it's more a curiosity skill that almost no recruiters value 
• For developers trying to learn ObjectScript - what was that process like? classroom training + practical exercises  What were the main challenges there?
  • In-person learning?  Y
  • Online class?  N
  • Documentation?  Y
  • What were your impressions? almost all tried to map the previous experience to ObjectScript the concept of variable scoping + global variants shocked them they had massive problems following CSP=>CLS=>MAC=>INT=>OBJ  
• How did it compare to other languages? They were looking for public examples. Support of SQL was often disappointing compared to market leaders
• What would have made it easier to learn? pushing it to universities and technical schools instead of retreating and leaving that space to competitors
• How long did it take to learn?    basics 2 weeks - guided development 3 months
• Was it longer/shorter than expected?  Just a predefined schedule  
• Easier or harder than expected? Hackers and Investigators had a clear advantage Formal thinkers suffered the most.

The main environment was a huge application Upgraded from
MUMPS to ObjectScript with a lot of old-style code using enough
ancient tricks that might "Code Quality" drive crazy.

Are there any other comments you'd like to make about the hiring
or training process that might help us improve?

My personal prerequisites list:

• understanding Java (:= Object concept)
• understanding SQL DBs
• willing and able to think across the fence
• rather exploring new ideas than varying old models
• courageous enough to explore and fill white space on their skills map
• digging into details

Besides the technology item:
being rather an explorer than an administrator

I've trained about a dozen junior to senior developers in InterSystems (and predecessors) technology.
ObjectScript is the least you should worry about. Any developer who can't grasp that isn't worth hiring.
It's the IRIS ecosystem that is completely different from 'traditional' development. Globals, Namespaces, (Remote) Databases, Mirrors, Productions, Jobs. All concepts that are not found outside the InterSystems (MUMPS) realm.
If people get their head around that, they either love it or hate it.