I'd divide the guys into three categories.
The fumbles: Roslovic, Petan, Vesalainen. Three high draft picks, all with good offensive potential for which they were picked in the first place. Yet, we refused to give them long looks in a scoring role - this is unacceptable. If you need fourth liners, you can get them free of charge from basically anywhere - we cannot waste high picks like this without even giving them a fair chance. Niku fits this profile too, aside from the high pick part. As for Roslovic, even with the great top 6 we had during his ELC, we always had the option to spread out the wealth and give Jack a better shot - we didn't do it. That's on us.
As an aside, Heinola looks to be a lock to join this group. FFS.
The what-ifs: Basically everybody aside from the aforementioned three. I understand and agree that there are not enough roster spots to test out everybody in a scoring role, but you could do it in moderation. Our choice has been to not give basically anybody a chance above the fourth line / third pairing, which has led to losing actual NHL level players like Harkins, Kovacevic and Eyssimont, unlikely as it was that they would amount to much. We have had several 21-year-olds showing good promise - those are attractive assets to have. If you wait around for a couple of years, suddenly they aren't young and flashy anymore. They're closer to UFA, they might even get irritated about the lack of opportunities. You can't do that to everyone in the pipeline, like we have done for too long now.
The goalies: With goalie prospects like Comrie and Berdin, we might have missed out on something at some point, but given that it takes 40-50 games to get a good read on how a goalie performs, I can live with that. Unless you stumble into that opportunity as a result of an injury or something (à la Hellebuyck in 2016), you simply might never get it. Can't really fault the organisation for that, especially with the #1 role having been locked up for seven years now.