The integration of Distributed Energy Resources (DERs) in distribution networks comes with challenges, like power quality concerns, but also opens up new opportunities, e.g., DERs can offer competitive energy prices for final users by leveraging time arbitrage. A suitable method to fully exploit such opportunities is to compute the optimal DER schedule, either with a full three-phase network model or a more computationally efficient one-line equivalent. This paper presents under which conditions a one-line equivalent can and cannot be used to properly represent a modern and unbalanced power distribution network able to dispatch high levels of DER penetration optimally. Results show that one-line equivalents might be helpful when the problem objective function limits the amount or there are not enough DER counterflows. In general, a safe system operation and the impact DERs might have in the network depend heavily on the optimization objective and DER penetration level.