First, the only person/people who could give a REAL answer, is Ub, or people close to him that he confided in. Anyone else – even “Disney historians” are full of hot air if they pretend to know what Ub thought. (OR, if there are credible documents in Ub’s own handwriting that clearly spell it out.)
That said, I don’t think Walt took advantage of Ub. I think the industry was so new, and they were young enough, that the adventure and thrill of trying something new outweighed any potential ‘abuse’ by Walt.
Also keep in mind that Ub quit working for Walt when Disney lost Oswald. That could have been just a financial choice by Ub, or it could have been the result of years of frustration that things were going as well as they could have – and Walt would have been to ‘blame’ for that.
In general, I don’t believe most intelligent people get ‘taken advantage’ of… and I believe that Ub was intelligent. Smart people are making choices to further their career or endeavour, while… not-so-smart people need to get burned a few times to become smart.