The main assumption of behaviourism is that we are born a blank slate and all behaviour is learnt from the environment; and It focuses only on external factors that can be objectively observed.
Because the experiment’s behaviourists conduct focus only on observable behaviours, behaviourist theories/experiments can always be falsifiable meaning they can always be potentially proven right or wrong. However only focusing on observable behaviours makes the perspective reductionist because factors such as cognitive processes and biology are excluded. A further strength of measuring observable behaviours is that data is easier to quantify and collect making carrying out statistical tests easier.
A weakness of behaviourism is that many of behaviourist theories have come from being tested on animals; for example skinners experiments on operant conditioning using pigeons. This makes the findings less valid because humans are so much more complex than animals; animals only rely on basic natural instincts: food, reproduction, survival. So the research may not actually be applicable to humans. Nevertheless, carrying out research on animals means that important theories can be tested that would otherwise be too unethical to test on humans. As shown by Skinner’s research on operant conditioning that involved pigeons locked in cages and first starved.
Another weakness is that because behaviourists believe all behaviour is learnt, sometimes behavioural therapies for disorders cannot actually cure someone, only remove certain behaviours caused by the disorder. For example if someone was suffering from depression, a big part of depression is how the person thinks but the behaviourist perspective may not be able to change the way someone thinks because it ignores cognitive processes; meaning the actual underlying cause for the disorder is still present. However for disorders that are learnt such as phobias behaviourist treatments such as classical conditioning have been shown to be very beneficial.
Overall even though behaviourism has been shown to have a lot of advantages and is very useful to psychology as a whole it’s disadvantages are quite prominent it may be better to use a combination of approaches to give a more holistic view.