If you stick to a basic, non suspended style of bike, the lower priced ones can be completely adaquate, but it helps to know the difference between a decent, good value bike and a " Bike Shaped Object".
The major brands don't make BSO so you are safe with them.
Shimano components are good even in the lowest price range. BSO usually fit lesser brands.
BSO frames usually show very poor alignment. The forks may point in different directions to the frame where you would expect them to be in-line.
Full Suspension BSO should always be avoided, but I have ridden some skip rescue discount brand bikes with eg Microshift gearing. They work, the brakes are not as reactive as better models, the metal nuts and bolts are softer and easier to mash. The wonkey forks restrict use to easy riding, no big descents or exttext off road