Is it advisable to use LinearLayout inside ConstraintLayout in Android?

Is it advisable to use LinearLayout inside ConstraintLayout?

No... usually.

In general, the idea behind ConstraintLayout is that it allows you to position all of your children without having to nest any other ViewGroups inside the ConstraintLayout. As such, I would say that it is not advisable.

However, there are some things that a LinearLayout can do that a ConstraintLayout can't (mostly revolving around weighted spacing of views), and so if you need these particular corner cases in your layout, you won't have any option other than falling back to a LinearLayout.

how popular is the ConstraintLayout among the developers?

ConstraintLayout is relatively new, but it is quite powerful and certainly something that you ought to familiarize yourself with. It won't always be the perfect tool for the job at hand, but it will often allow you to easily create layouts you would otherwise spend hours on.

I can't speak to widespread adoption statistics, but I can say that I've seen tons of questions on this site about the correct usage of ConstraintLayout, so clearly devs around the world are starting to work with it.

As of the 2.0.0-alpha5 release of the constraintlayout library, it's now possible to declare a Flow virtual layout element within your ConstraintLayout which (as the name suggests) determines how referenced items are to flow (e.g. vertically, horizontally) within the ConstraintLayout. So it's no longer necessary to declare a LinearLayout within your ConstraintLayout.

For example, if you wanted items within your ConstraintLayout to flow vertically, you'd do so like this:

<androidx.constraintlayout.widget.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/textView1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="I am the first TextView" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="I am the second TextView" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:text="I am the third TextView" />

    <androidx.constraintlayout.helper.widget.Flow
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:orientation="vertical"
        app:constraint_referenced_ids="textView1,textView2,textView3"
        app:flow_horizontalAlign="start"
        app:flow_verticalGap="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

You can play around with the app:flow_ attributes in the Flow element to achieve different flow behaviour. For more information about the Flow element, refer to the release notes here. For an example, see here.