what is the alternate for it?
You should only use ViewModels, It's the recommended approach. Create Strongly typed views. It would make your code cleaner and easy to maintain.
What extreme problems will be there?
Issue with Viewbags/ViewData is compile time checking. ViewBags are just dictionaries and you won't be aware about any issues until run time and thus it makes your code error-prone. So always try to go with ViewModels.
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