What's typically wrong with cheap mountain bikes?
A fully for 700€ will be horrible. Bad suspension, heavy, few gears, bad wheels, bad tires …

At least get a hardtail where 700€ will buy you a halfway decent front suspension (where the lock-out actually works) and okay-ish weight. With wide, tubeless tires and good riding skills you’ll barely notice the absence of a (bad) rear suspension.