I was thinking about the tier lists this morning and I had a bit of a revelation, and following that a concern.
In terms of both the restrictions and benefits, the existence of the tiers may have a profound effect on future model development within a faction.
Now, it's also true that new models are profoundly affected by what already is available. This is to be expected with model synergies. But the tiers add a whole new layer. Most of the tier allowances are restricted by statuses like character (for warbeasts/warjacks), named types (like Legionnaire), or an ability (stealth, flight, incorporeal).
What that means is Privateer Press must balance vs. both open faction synergy (how everything works with everything else) AND potential tier based inclusion.
Let's take a few examples - Saeryn's tier 4. Because this tier exists we could never see a Heavy Nephilim with flight, as you could easily add 2 of these +1 light Nephalim and get the benefits of both tier 3 and tier 4 using the same models (thus reducing your investment needed to get the tier benefit). It might not be in their plans to ever do it, but it does create a design restriction that wouldn't exist without the tier benefit. Similarly, you'll likely never see a 7 point Nephilim or a 4 point Nephalim, since both of those would allow even more edging of points out of the tier system.
Or something like Thagrosh's Tier 3 benefit - Because this benefit exists, you'll probably not see a legionnaire UA that would grant AD. Again, not something they may plan on, but something they probably couldn't get away with due to the tier benefit.
Looking through the tiers, I could probably come up with a ton of examples of design restrictions that result from the tiers.
From the other direction - because the tiers exist now, many future models that don't already fit by one of the inclusions that exist in a tier have to be retro-fitted to be allowed into a tier, barring some kind of tier re-release. Also, because of the named/typed benefits and inclusions, every newly designed model must be considered and naming conventions avoided in order to prevent it or specifically to include it in a tier. Not a big deal, obviously since the naming conventions are purely at PP's discretion, it's just another factor.