Regarding breathwork, I've no experience at all, I've only studied how slow breathing changes your HRV (heart rate variability). Many years ago there was a lot of talking about a form of "heart meditation" so I studied it a little bit. You can start reading from here
to make you an idea. Their claims are a bit exaggerate, but they are true about HRV.
Regarding the so called "mind-machines" (flashing goggles) this is my experience:
I've one of them (model "Sirius", a very basic one) and I can tell they are half effective in what they promise. They are NOT good in putting you in an altered state of mind of any sort, but yes, they are good at delivering visual hallucinations under the form of fractal patterns. Very strong and coloured geometric patterns, swirling and rotating around a pivot point located in the center of your visual field. The colors are very bright and saturated and sometimes the visual field expands beyond your peripheral vision with an amazing embracing-all effect.
The model I have has an option for alternating flashing light from one eyes to the other (instead of simple on/off) which changes slightly the perceived fractal pattern (now more "striped").
You can adjust the frequency: 5-7 Hz produce big red and deep blue fractals, around 10-15 Hz you get yellow/blue. The higher the frequency, the less fractals because I guess there is latency in the retina. You can go up to 25-30 Hz, beyond this the fractals become a tiny white flickering light.
You can use machine's programs where frequency is changed according to a schedule, or you can just have a fixed frequency running. Those machines have also binaural beats via earphones, but I usually leave audio turned off (because I find binaural tones disturbing).
When you end your flashing session, if you stay with eyes closed for another 5-10 minutes, you see that the "blackness" of your eyes has changed temporarily, now it's more even and plain like if it was washed. But that effect lasts no more than 10 minutes.
As I said in my previous message, I've recorded my EEG while using the goggles, and there is a strong trace in correspondence of the flashing frequency. This tells you that brainwaves are effected, but in my experience this does not result in a changed state of mind.
I've also briefly studied the flashing goggles after the use of Salvia Divinorum: the fractal patterns are somewhat distorted, they are more "salviesque".
Another thing that I remember is that flashing goggles are much less effective if used on the evening (e.g. before going to bed), I don't know the exact reason of it.
In conclusion, flashing goggles are amusing the first 2-3 times you use them, but soon you get heavily bored. Mostly because the flashing light itself is disturbing and it's not tolerated easily.
I've also some experience regarding neurofeedback machines and biofeedback (GSR/temperature), if you like we can talk about them too.