from technical prespective, with the Qlikview converter it's fairly easy
to move the data , object and script
the real issue, is clients tend to look for Qlikview in qlik sense
and look for the same capabilites, and a lot of time are frustrated with missing capabilites.
so for me, each conversion product start with selling the client the idea of self service
and the need to redisgn both the visual display and the approch on how to use the app
if the client buys into this than the transformation is easy.
Qlik Sense's responsive design and rewritten visualisations certainly give it a different look and feel. The biggest hurdle I found was encouraging people, both users & developers, to work with the product and not against it.
I would suggest where possible to do everything Out Of the Box and avoid bespoke extension visualisations and mashups. Some people are tempted to try and recreate QlikView Apps in Qlik Sense that are identical to QlikView - but if the objective is to exactly replicate QlikView then I would question the decision to migrate when you have a QlikView that is already identical to itself. I'll repeat myself: Work with the product and not against it.
When you have your base migration to Qlik Sense completed and Live, then it is the time to really exploit QlikSense. Having initially avoided bespoke extensions and gone OOB, now you will have enough experience of Qlik Sense, and your migration to it, to start exploiting bespoke Qlik Sense extensions with the hindsight you now have.
I believe the real strengths of Qlik Sense are its responsive design and rich API's to create bespoke extension visualisations and mashups. But to quote an old cliché you need to do the right thing at the right time for the right reason.