Grandland is a nice car, newer Opels are still German engineered and designed but they do share a lot of parts with PSA. That in it self is nothing new, even under GM they shared with PSA. Fiat and may other makers.
Yes, resale values are lower than other brands, but that is where you can score if you buy used. I picked up a nice Astra J in top level trim used for R170k a year ago. Way more car than you get in say a current model Corsa F which costs more than double the price.
My current Astra is the 5th Opel I have had and my 3rd Astra. If you look after these cars they will go a long way. I can say the same about the single Peugoet I have had the pleasure of owning, with the caveat - do not fit pirate pumps from China.