Esters can be used at higher levels than many might think. Indeed the first synthetic oils in the early 70s were 100% ester based without issues. The primary limiting factor is cost, and the fact that additives respond differently in many esters, requiring more development resources.
That said, seal swell can be an issue depending which esters are used. Some esters have very little seal swell characteristics (a little can be desirable) while others such as low viscosity highly polar esters can swell seals excessively. Personally I would not use highly polar esters at over 20% of the base oil blend unless balanced with some seal shrinking PAOs, although I am not aware of any seal problems in the field from higher doses. You also shouldn't need more than 20% to enjoy some of the benefits of additive solubility, seal balance, lubricity, low volatility and cleanliness. If you want to use more for a high performance oil, than PAO balancing is wise.
Water absorbance from esters is not a field issue. Most esters will only solubilize 0.2% water, and it evaporates off in use. Again it comes down to using the right product for the application. Esters are a very broad field of chemistry with very wide property ranges. The esters used in motor oils are selected specifically for this use, and like all ingredients are balance as necessary with the rest of the components.
