The demo has clear disadvantages.
Firstly, you shouldn't judge about trading on a real account by trading on a demo. These are two absolutely different worlds.
Secondly, trading servers are constantly under the supervision of an experienced team of specialists, providing uninterrupted operation of servers and the program.
Thirdly, there are different psychological aspects: you trade with virtual money, so take it not quite seriously, and trading requires a conscious approach and considered actions.
Another important difference between demo and real trading is the speed at which positions are opened. On a demo everything happens faster and always with the price you wanted, but in real life it is a little different. There is a concept of the first market price. But this is exactly the concept, the first market price has no clear definition or mathematical formula, moreover, it may differ in different companies. In case of sharp price movements, a gap may be formed. This happens when the next quote is very different from the previous one. In such a situation, on the demo the order will be executed at the price specified, and on the real one - at the first market price. There are almost no cases when a client gets a new price on a demo. In reality, this can happen due to the fact that quoting on a demo is performed by a computer with a sufficiently large allowed deviation from the current price. And on a real account dealers monitor strong price deviations in the request from the current market quote.