For statically indeterminate trusses, which method is better? Force or displacement method? And why? I read somewhere that displacement method is not suitable for trusses but I must verify.
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Depends on your definition of "better" and on the specific truss being analyzed.
The displacement method is useful in that it is agnostic to such things, statically determinate and indeterminate structures are identical, as far as the method is concerned. Obviously, indeterminate structures will have more complicated solutions, but the basic working is all the same. This is why almost all structural analysis software uses the displacement method.
For structures which are only slightly externally indeterminate (by one or two degrees of freedom) and which aren't very complex (or which can be decomposed into simple sub-structures), the force method might be useful if you're doing things by hand.
For internally indeterminate structures... I honestly don't remember how to use the force method, so I'd certainly use the displacement method.
In case a refresher is needed on internal and external indeterminacy: external indeterminacy means you can't find the support reactions using only the equilibrium equations; internal indeterminacy means you can't solve for the members' internal forces using only equilibrium equations.
