Thank you for your accurate answer @Jack! It makes much more sense now.
So, I will need to convert the 'em' units to either pixel or percentage values to get the result that I'm looking for.
Can I ask one, possibly silly question... what happens if I would like to animate elements on a website which uses 'rem' and 'em' units by default (like Bootstrap 4), and considering that I use pixel values in the Tweens, would that cause any inconsistency in the design between mobile and desktop devices?
Cheers,