To keep cryptos in trading platforms implies several risks like for example:
- The platform gets hacked and funds oftenly are not insured.
- The platform can cheat you.
- The platform becomes bankrupt by mismanagement or unlucky circumstances.
- The TOS of some trading platforms don't allow to keep funds there without trading.
Also keeping funds on wallets is not without risk especially for newbies.
Mostly loosing private keys/seeds can cause loss of funds.
The best is to use more than one wallet and only keep money at trading platforms, which are active used for trading. Also use more than one trading platform.