– You need to come every other day to feed them, otherwise they die or differentiate. – You have to come on weekends too. – If you are not super super careful, some bacteria or yeast will eat them up. – Even if you are super super careful, some bacteria or yeast will sometimes eat them up. – In experiments, there are huge differences in the behavior of cells so you will have huge errors and have to repeat the experiments multiple times to prove your point.
+ You have to do it, so your other studies are tested on something alive and thus are proved relevant.
That's it. Nothing else to the + list. Cells are bitches. You have to nurture them and pamper them and sacrifice weekends, and then they die or behave unpredictably.