Java (v8) 390 276 bytes

public static Object[] f(final Object[]a) {
    List<Object>r=new ArrayList<>();boolean t=false;int n=0;
    for(final Object p:a)
        if(t=p instanceof Object[]){for(final Object q:(Object[])p) r.add(q);}
        else r.add(p);
    return(t)?f(r.toArray()):r.toArray();
}  

Just for completeness and all that. :) Can't say Java's code-efficient.

Java (v8) 390 276 bytes

public static Object[] f(final Object[]a) {
    List<Object>r=new ArrayList<>();boolean t=false;int n=0;
    for(final Object p:a)
        if(t=p instanceof Object[]){for(final Object q:(Object[])p) r.add(q);}
        else r.add(p);
    return(t)?f(r.toArray()):r.toArray();
}  

Just for completeness and all that. :) Can't say Java's code-efficient.

Java (v8) 390 278 bytes

public static Object[] f(final Object[]a) {
    List<Object>r=new LinkedList<>();boolean t=false;int n=0;
    for(final Object p:a)
        if(t=p instanceof Object[])for(final Object q:(Object[])p) r.add(n, q);
        else r.add(p);
    return(t)?f(r.toArray()):r.toArray();
}  

Just for completeness and all that. :) Can't say Java's code-efficient.

Java (v8) 390 276 bytes

public static Object[] f(final Object[]a) {
    List<Object>r=new ArrayList<>();boolean t=false;int n=0;
    for(final Object p:a)
        if(t=p instanceof Object[]){for(final Object q:(Object[])p) r.add(q);}
        else r.add(p);
    return(t)?f(r.toArray()):r.toArray();
}  

Just for completeness and all that. :) Can't say Java's code-efficient.

Java 390 bytes

public static Object[]f(final Object[]a){
    Object[]r=new Object[a.length];boolean t=false;int n=0;
    for(final Object p:a){
        if(t=p instanceof Object[]){
            int v=r.length-((Object[])p).length;
            if(v<0)r=Arrays.copyOf(r,r.length+Math.abs(v));
            for(final Object q:(Object[])p)r[n++]=q;
        }
        else r[n++]=p;
    }
    return(t)?f(r):r;
}  

Just for completeness and all that. :) Can't say Java's code-efficient.

Java (v8) 390 278 bytes

public static Object[] f(final Object[]a) {
    List<Object>r=new LinkedList<>();boolean t=false;int n=0;
    for(final Object p:a)
        if(t=p instanceof Object[])for(final Object q:(Object[])p) r.add(n, q);
        else r.add(p);
    return(t)?f(r.toArray()):r.toArray();
}  

Just for completeness and all that. :) Can't say Java's code-efficient.