I have a 2015 base model and 2017 MacBook. Never had any trouble with either keyboard and I do type a lot on them. I don’t have food/drinks while using it and give the keyboard a quick wipe down with a lint free cloth before closing the lid.
Having said that, buying used will always be a roll of the dice. One will never know the environment in which it was used previously. If you are able to test it out before buying, then I would be more confident if all keys are working and it looks to be in good shape. Of course, YMMV.