implications of using vue (or any other js frontend framework) as oposed to pure oldschool php development?

Coming from pure php frameworks, for a new project i decided to completely separate frontend from backend and i wondered what implications this would have compared to the former way of working?

what i mean is that using something like vue you could simply move a lot of the server load to the front end.

Just get data, apply logic in js frontend and send the computed data based on user input back to the php api for storage.

But since i cant trust any of the data being send back to server the (since anyone could send anything back by manipulating the DOM and thus bypassing restraints)... it seems that all the constraints build in on the front end need to be double checked on the backend.. again... no?

this seems like double the work.

or is this not the way to approach this separation? or is it just the tradeoff on having such a seperation?

Any thoughts on this form folks who have had experience with this? Also, any other pitfalls i should be aware of?