First of all, don't listen too much to what everyone says (including me). I really like spiritualist, it's very flavourful but it is indeed not very strong. You may need to optimize your character more than the other players to be as effective as them. Summoner for example will almost always be stronger.
On the other hand you don't always have to care about how powerful something is. As long as it's consistent with what you decided with the DM and the other players.
I think priest of the fallen is almost an unchained spiritualist as it gives you a lot of interesting stuff with few drawbacks if you want a combat oriented spiritualist. It is also compatible with fractued mind (and scourge but it's highly optionnal).
When you combine everything, you'll see that some foci are more effective at lower levels while others are stronger later. Being able to totally customize your phantom twice a day provides an incredible flexibility at the cost of a ton of research and bookkeeping.
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As a final note, a lot of the guides you will find on the internet are terribly outdated and paint a very negative image of the spiritualist as it was before it received some deserved buffs (e.g. the priest of the fallen).