If you make a map using Reforged models you still can make it playable in SD mode (by turning on the corresponding option in the World Editor). However, in SD regime the Reforged models are missing and I think this (or something else which is similar) causes a lot of desyncs when you play with people having different graphic regimes. I tried to play my map, which uses Reforged models and is compatible with both SD/HD modes, with my friend, who has only Classic. At every time he was desynced (or was getting an error) after or during the loading.
Maybe the best solution is to associate with each Reforged model the “closest” Classic one. For example, there are lot of new golem models in Reforged. It is better to show the classic Rock golem model than empty if the new golem models are used.