There is a lot more to a welder than just a transformer and rheostat these days. Especially some of the fancier TIG units, the electronics allow very precise variation in how something such as the AC output can be steady, or pulsed; biased with more of the sine (or square) wave towards the electrode or the work and a great deal more. Also, modern digital controlled welders are a heck of a lot more efficient than old all electric machines. For instance, my 30 year old AC/DC/Tig/Mig/Stick machine is only 200 Amps, but will trip a 60 Amp breaker on the 240VAC side while pulling not much more than 150 DC @ 40V. New machines WAY better than that (you will note that you can pull 140A out at 40V or so with only 120V 20A breaker feeding it on a Miller 211 or Lincoln 140A).
As has been mentioned, they all break. BUT: in decades of both heavy hobby and commercial production with several welding machines, I have only had to do one repair (and that was a big one). They are pretty low maintenance machines and you can really get your money's worth out of them. BUT: I do recommend sticking with major brand names - support if you need it won't be there for discount junk. In time that could include things such as consumables (liners, gas cups, tips, tip holders, etc.). So in North America, that means Miller or Lincoln (I have, for instance, a really nice and once very expensive ESAB machine that is now an orphan - zero support).