More difficult how? Its essentially the same thing with a grid of icons, it couldn't be more easy. In fact I showed it to my 70 year old mum and she thought it was still the old Apple TV box!
There are many things that make it more difficult vs the ATV3.
Using an IR remote, the same one they use with their TV and Cable box, was simple with the ATV3, but now there is no consistency between the video apps for the buttons. Example: to pause and play on ATV3, you press the select button for HBO, Netflix, Hulu, You Tube, and all the other video apps. With the ATV4, pressing select can do many different things depending on the app that is used.
For browsing genres on Netflix on the ATV3, there was a menu that said "Genre". I still can't figure out how to do it on ATV4.
Doing a search on ATV3 vs ATV4.
These are just a few things, let me know if you want more.
My parents are around the same age as your mum, and I think it is nice that your mum likes the new ATV4. Although, I do not think the new UI has a lot in common with the ATV3.
But, I know my parents, and if they were forced to use the siri remote, which, right now they would, then they wouldn't use the device.