With the canon 50mm F1.8 vs 1.4, the 1.4 is what I have. The 1.8 optically is the best of all the 50mm in canon's lineup, even over the 1.2L but it is the least frills of them all. The 1.4 is the best compromise between the build and optics(with the expectation of the 50mm F1.8 mk I, it has better build in my opinion, just lacks FTM focus, and is the same exact optics as the mk II). It is worth it for me just for the overall build and mostly because my 50mm F1.8 mk I is having focusing issues and just hunts unless I clean the contacts regularly.
The issue with the AF system is because canon did not use a ring USM for focus and instead used a micro USM, which is basically a micromotor AF motot that works on the principal of USM motors. And as such, to get FTM focus with that motor they had to create a complex clutch system to allow it. The system works very well and is very reliable. The issue comes from that the lens elemets extend as it focuses towards the MFD. So if you were to have the lens extended and happen to hit the lens hard enough, you will damage the gears internally. This lens has the issue more then any other and is why people think there is an issue. But in reality, if you hit any lens hard enough it will break, and this is handled simply by having a hood in place and making sure to store the lens at infinity(which also makes the lens smaller).