I think they will be copying reformists on most issues (Kurt Schumacher have reformist economic plan), but also support Reichsbanner, any anti-far right things and bunch of rally for republic and democracy.
In my opinion, advisors from the Neorevisionists just too weak. They show up in mid-late game, but they can give you... Rally for democracy, event on Reichsbanner and just buff on Iron Front... This is simply not enough.
My options for fixing this
1. Give Mierendorff the ability to beat the Nazis rally through our paramilitaries, which will reduce their popularity. Add this feature only after the Iron Front is formed
2. Give Schumacher the ability to strengthen the Reichsbanner, but for free. Also, instead of the reformist plan, give him the opportunity to increase the number of police in Prussia by 40 thousand, and not 20 as in the usual event for recruiting police in Prussia.
3. Give Leber the ability to rally without the possibility of disruption. The rally is not for democracy, but simply an event for the rally.
4. Give Mierendorff more buff to his ability "Warning about the Nazis". I like the idea, but impact just too weak. That ability can, for example, reduce dissent of all wings — like Wells, but 2-3 times stronger. Or/and improve relations with all parties, including the communists (Excluding NSDAP and DNVP), and reduce dissent in the coalition. Improving relationships with parties can also be improved, but not by much - 1.5 times, for example.
In other words, make them stronger than other advisors. Because they appear much later. So they should have some abilities that only they have, which give us a significant advantage over those advisors who are available to us from the very beginning of the game.