Check what form each reward takes. MEXC advertises $10,000 in new-user rewards, but the usable part is roughly 40 USDT in vouchers. Bitget's welcome pack, advertised at up to $6,200, is futures vouchers with little value to a typical new user. BitMart's 13 USDT, CoinEx's 70 USDT and Gate's 118 USDT starter package are all fee or futures vouchers.
A futures voucher lets you open a leveraged position with the exchange's money. You may keep a profit, but you cannot withdraw the voucher itself, so using one means taking leveraged risk to earn a small amount. For most new users that is not worth it.