SVector is only efficient when the length of the vector is known at compile time, because the length is a part of the type. If you have a runtime push! loop, where the compiler doesn’t know the number of loop iterations, using SVector will be type-unstable and hence slow.
(The reason why something like SVector is not the default array type in Julia is that they are much less versatile than Array … but for the specialized cases where SVector applies, they are fantastic.)