I saw lot of companies advertising in online or in TV that we can reduce belly fat by using their machines. Is that really works , Is there any disadvantages ?
In short, you can't target fat loss, and what these types of companies are claiming is purely marketing. Sure, if you use a machine like this you will burn some calories, and if you burn more calories than you're consuming you will reduce fat (on your entire body) and get some results.
The thing is, these types of machines aren't as effective at burning calories as doing cardio (running, cycling, swimming) or doing heavy compound lifts (deadlifts, squats). They will burn a lot more calories, and combining this with some ab work like leg raises, crunches or planks will give you more results than these machines.
Also, abs are revealed in the kitchen, track your caloric intake and make sure you are burning more than you're consuming. And if you want to build some muscle, you have to make sure you get enough protein.
There actually are a few medical things you do (beyond liposuction). Look into CoolScuplting and SculpSure. Both are FDA-approved. No experience with CoolScuplting, be we have a ScuplSure machine at our medi-spa, and I've seen it work firsthand. Up to a 25% reduction in fat for treated areas. Currently approved for abdomen, flanks, back and inner/outer thighs. Uses lasers to burn the fat away. Usually takes about 12 weeks to see full results.
That being said, for true body-recomposition, the comments and answers above hold true - abs are (mostly) made in the kitchen. Get into a calorie deficit, and stay there. I say mostly because without direct ab work, abs won't "pop" when you lose all the weight.