Starting on steam definitely has some advantages.
With the same inputs you can generally get the same result whether it's steam or glass. The difference is that steam requires your brain to do a lot of the processing of the inputs, whereas the glass does it for you.
The Law of Primacy states:
Primacy, the state of being first, often creates a strong, almost unshakable, impression. Things learned first create a strong impression in the mind that is difficult to erase. For the instructor, this means that what is taught must be right the first time. For the student, it means that learning must be right. “Unteaching” wrong first impressions is harder than teaching them right the first time.
Learning to become a pilot really consists of one primary thing: building blocks. We flight instructors know that a student can't accomplish X unless they've already mastered blocks A, B & C. We know that they can't complete C unless they've completed B and A. And so on. These blocks get incredibly burned into a student. They get really good at executing these blocks.
Steam gauges require that pilots do a lot of interpretation of information and context. Really too damn much of it. That's why we got glass in the first place. The engineers have put a LOT of work into displaying information to a pilot as efficiently as possible. Easier. That's a good thing really.
Problem is when you put one of those glass-only pilots into a steam gauge aircraft things get ugly. They now are required to do a LOT more interpretation throughout all the phases of flight. The building blocks they were originally taught, that have served them well, and kept them alive so far, suddenly don't contain what they need. They need to learn a considerable amount, if gauges fail, they actually have no training on how to recognize it. They don't get big X's and computer warnings. They have to be the computer and figure it out... It can certainly be done, they can transition, but it'll never be as programmed into them as the other things they learned on their first flying lesson.
Of course a steam only guy can't safely just jump in glass and fly on a 200 ft ceiling day, especially if he had never operated a garmin product or an IFR GPS. Transitioning between either types should be taken seriously.