Hi, the following comes from a Pathfinder lover who has bought many books, played and DM'd several campaigns.
First off, the whole concept of taking out of class options instead of advancing in two classes separately is great.
- It gets rid of the "1st level in another class is better than 7th in mine" problem
- Advancement is more streamlined
- It's practically impossible to make something that is off flavor
What I found to be one minor flaw, is the balance.
Now I know that taking outside-of-class features should be somewhat worse than your in-class alternatives. Sure.
The problem is, level 6 and above we have "Gain one SUBCLASS feat. For the purposes of meeting its prerequisites,
your SUBCLASS level is equal to your actual level."
Pretty strong eh? However, the current requirement arrangement is really rough for low level characters.
If the adventure starts at 1st lvl and a player has a character concept that requires multiclassing, story-wise, they're screwed. How can a barbarian become a priest over a week? (My favorite character of all time is a half-orc barb that is also a tribal cleric of some god of strength, varying by setting).
Another thing is that often you must trade 4th lvl feats for 1st level features.
The solution is pretty simple.
1) Make Dedication feats accessible from the 1st level. Some things may need to be tweaked around it, but it's well doable.
2) Make 1st level archetype class feats and features accessible on 2nd level. Nothing that scales obviously.
3) Make other feats and features accessible as though the SUBCLASS level was HALF your character level.