My main concern would be seal compatibility, although that depends on which POE is used (there are many!). Cost is another factor - the best formulation in the world is of no value if you can't sell it. Adding PAO or Group III+ will help balance the seal issue and lower the cost.
Car engines do not need a 100% POE based oil as they do not run hot enough to utilize the key benefits. POEs will deliver diminishing returns in a mild environment as their main attributes are very high temperature stability and low volatility. Their other benefits, such as high lubricity, natural detergency & dispersancy, additive solubility, and cleanliness, can be achieved at lower doses in car engines.
POEs are rarely used anymore in synthetic engine oils due to the high price, and less need in Group III based products. When they were used (with PAOs) the dosage was generally 10-20%, the exception being Redline who state their oils are mostly POE based. The Mobil 1 line has largely substituted ANs for POEs in their Group III+/PAO formulations. ANs will balance seals and solubilize additives, but do not offer the other benefits of POEs to the same degree.
The two biggest drivers for most motor oil formulators are specifications and cost. High doses of POEs do not fit either.
Tom NJ