The engines are larger than previous models and had to be moved forward, which causes it to (potentially) stall, so they implemented an anti-stalling mechanism called MCAS which relies on a particular sensor, which can malfunction potentially. They also didn't inform pilots that this system even existed which means they have trouble diagnosing the problem and will potentially only make it worse by trying other things.
Of course that's all speculation since we don't know much about this particular crash, but that's the main issue with them.
Of course that's all speculation since we don't know much about this particular crash, but that's the main issue with them.
Source: https://www.youtube.com/watch?v=zfQW0upkVus