GW have struggled with the rules for Daemons for decades now.
In 6th edition, daemons we’re the red-headed step child of WFB because their ward save was negated by magical attacks, they couldn’t use the army standard bearer re-roll failed instability checks, and the entire unit disappeared if you rolled over it’s unmodified leadership. As a result, you never saw them outside of a gimmick list until the Storm of Chaos book gave them both a true ward save and the ability to re-roll instability checks. Units would still disappear if you rolled over their leadership on an instability check and internal restrictions meant you were generally locked into building mono-god lists, but at least you could field a competitive army. At the time I fielded a Nurgle army and it was strong, but not unbeatable.
In 7th edition, daemons were the apex predator of WFB because in addition to the true ward and instability re-roll, GW removed the internal restrictions on army building which allowed people to min/max the best units regardless of which ruinous power they were associated with. Coupled with some really good daemonic gifts, daemons regularly won tournaments. At the time I fielded an army with one each herald & core unit from each power & a couple of special choices. It was not an optimized army, but it still did rather well.
I only played one or two games of 8th edition, so don’t have any first hand experience with how daemons played at the time, but I gather they were pretty strong (but not as strong as during 7th edition).
With the Old World, being as they’re a legacy faction, I haven’t played any games with my daemons because I’ve returned to my first love - the Empire. With that said, having read GW’s legacy PDF, I had no issues with the army building restrictions, but the one rule that immediately stood out was the reversion of the daemonic save to being negated by magic. That is bad because daemons are expensive and most do not have an armor save. With that said, being able to field an entire army with a full-time 5+ ward save probably isn’t that great for the game, either. Had I been involved in the development of the Renegade Pack, I would have suggested making it so magical attacks reduced the ward save to a 6+ instead of negating it completely. Obviously, people need to get some games in with the Renegade Pack rules before we’ll know for sure.