Players like Soldado failed in the EPL not because they are shit but because of mismanagement. Iago Aspas didn't even look like a League Two player at Liverpool and only little better at Sevilla afterwards, back at Celta Vigo he's one of the best players in the league who annihilated Barcelona pretty much single-handedly.
Of course players are also responsible for adapting to a new environment but if the club decides to spend 30m on a striker from a different country they better make sure to use and include him properly, it's not like Soldado had a difficult character or shit work rate at Spurs, he just wasn't used properly at first and then lost all confidence as a result.