Not to sound sarcastic, but some were done by competent gunsmiths, and some were done by what we used to call "shade tree mechanics". In other words, some were very nicely done and some were done with a hammer, pipe wrench and ill fitting screwdriver.
As far as reliability...product was only as good as the person performing the work.
If you are looking at one, there are so many 44's out there now that I think it would be a waste of money.
The advantage to having a proper conversion is knowing that the revolver is dimensionally perfect in regard to cylinder throat and bore diameter. Small body reamers also cut tight chambers that do not work the brass as hard, compared to the mass produced product. In this day and age with all the sophisticated CNC machinery it's still shocking how many revolvers are produced with cylinders that are not bored properly. But I wouldn't buy one of these revolvers unless I knew who made it and the seller could prove it. Most modern .44 Magnum revolvers have decent chambers. Usually the conversions (like the one I have) are done in .45 Colt.