For people who like it, no downside at all. Personally, I have tried it (repeatedly) but find I neither want it nor need it. I am a really good coil builder and I can gain the control advantages of TC by building a coil yielding them naturally, without requiring a computer circuit to handle the matter for me. I don't even need a VW mod; I can do it on a mech. For me, I've been doing it so long now it's no trouble at all. In fact, it would feel odd to build a coil for anything without considering thermal mass and ramp time and surface radiated power and so forth. For others, though, this is more effort than they care to put forth; some (most?) would rather use drop in coils and not build at all.
While there isn't much in the way of a downside (now Ni200 wire has been rightfully consigned to the dustbin of history) there are plusses and minuses. Assuming you already know why you might want to, the minuses are basically you get what you pay for: if you want really good TC, you'll part with some solid bucks for the right board. And, to take advantage of the top-of-the-line board you just paid for, you'll find there's a bunch of adjustment settings to make to get it tweaked just exactly so, and that this tweakage varies by juice and vape power range. Change juices? Change the settings. Use the same juice at 50 watts and 100 watts? Different settings. Once you get it locked in it's easily repeatable-- good boards allow you to save a certain number of settings, and you can also write them down-- but finding it can require as much finicky fiddling as me building a coil. This is, however, only if you want to obtain the maximum flavor you can get out of your juice, the optimum mod and battery life utilization, and if you feel like futzing with it. If all you want TC to do is stop getting dry hits, a cheap board and pretty much any old middle-of-the-road settings will do.
While there isn't much in the way of a downside (now Ni200 wire has been rightfully consigned to the dustbin of history) there are plusses and minuses. Assuming you already know why you might want to, the minuses are basically you get what you pay for: if you want really good TC, you'll part with some solid bucks for the right board. And, to take advantage of the top-of-the-line board you just paid for, you'll find there's a bunch of adjustment settings to make to get it tweaked just exactly so, and that this tweakage varies by juice and vape power range. Change juices? Change the settings. Use the same juice at 50 watts and 100 watts? Different settings. Once you get it locked in it's easily repeatable-- good boards allow you to save a certain number of settings, and you can also write them down-- but finding it can require as much finicky fiddling as me building a coil. This is, however, only if you want to obtain the maximum flavor you can get out of your juice, the optimum mod and battery life utilization, and if you feel like futzing with it. If all you want TC to do is stop getting dry hits, a cheap board and pretty much any old middle-of-the-road settings will do.