I think the bowling machine is best left for shot grooving. I never find it much help in picking up length / getting feet moving as it is too easy to tell the length of the delivery before it is even bowled. (once you have faced it a lot you will pick up small differences in the alignment of the 'hole' on the machine and will know if it's short or a half volley before it comes out)
If I want to practice picking up length and getting the timing of my foot movements correct then I prefer to face either real bowling or someone using a sidearm (or even throw downs).
As someone who has self taught and mainly from using a machine I'd say it does help you learn to pick lines and lengths better than facing a real bowler. When grooving a shot your brain learns the length/line as well so even when it comes down at 80 say you do react correctly, it's surprising but it does work. Now, I'm. It saying it doesn't ah it's faults. It I get far less now from facing bowlers than I do a good session on the machine. Indoor game or machine season ?? No contest, machine session is far more productive.
Same with pro, train on 60/65 but do the odd session at high speed just so you have seen it. You will react anyway so it's just for your own comfort really. I find 60 far harder than 75 for instance, why, because one speed you have to hit then ball and the other just time it. Exactly the same as I find in real games, 'quicks' are far easier than your slow/medium plod who just drop it on a spot etc