Why was it bad?

Managing skills and techlead and IC skills are pretty different disciplines.

Being 50/50 makes it hard to advance/develop in either one of them significantly.

The biggest issue is that management requires a lot of "wasted time" paying attention to whats going on around you and IC skills require a lot of "heads down time". It's a big fight between those two modes.

I've done it at a startup but it required doing most of my IC work after hours. Which isn't that sustainable.

It was terrible because the "managers" had very little training which made them mostly useless and a legal liability to the company in regards to employment law cases. In many instances they weren't even on the same direct team but an adjacent team, so rhey hahd very little interaction. This completely invalidated the premise that a technical/coding manager would be a better mentor since there was never any time for it. Of course the company paid them the same rate as the senior devs that weren't managers. I'd say at least 50% of the first year cadre left the company or reverted to a regular senior dev after one year or less. Most divisions of the company don't use this model now. The only real reason they did it was because Google did it.

It’s the only point in one’s career where you’re expected to do both programming and managing and it’s hard to do both at the same time and at a good level.