One downside I can think of is that some ambitious engineers might not like being unable to make their mark on a product with an idea they have. For what it's

One downside I can think of is that some ambitious engineers might not like being unable to make their mark on a product with an idea they have. For what it's worth, I've known too many Apple engineers decamp to Google, et al., after they got sick of the top-down management style. Google is, of course, the other extreme where products can essentially be the external manifestation of an engineer's effort to get promoted.