This seems the kind of exploitable dip LU designers wanted to avoid.
They specifically removed Action Surge for this, as well as changing Eldritch Blast from cantrip to a class ability.
I'm wondering if this should be reported as a bug/exploitable feature
Action Surge and Eldritch Blast were two big sources of "cheese," yeah. Blog post discussing that:
Keeping it Classy: Updated Core Classes in Level Up — Level Up: Advanced 5th Edition (A5E)
Honestly, it's not that much of a problem, though. With Divine Smite scaling by herald level rather than by the level of spell slot you have access to and being limited in use by your proficiency bonus, all that multiclass really represents is a herald who is better at raw fighting skill than supernatural stuff. There was a bit of discussion about it among the designers before everything went live, and the consensus was that it really wasn't a problem. In order to fully get the benefit of the fighter's expanded maneuver access, you'll want more than one level of fighter anyway; the exertion pool
is nice, but heralds don't get very many maneuvers. It's not a bad choice at all if you really like using maneuvers, but it's not anywhere near the broken nonsense a sorcadin was in O5E. Or, for that matter, a fighter 2/paladin X was. And unlike the O5E fighter, you can't break the action economy any more, so that's less of a concern.
You do wind up with a bit more versatility and staying power, but you also slow your advancement into things like Empowered Smite, Divine Recovery, more smite damage, etc.
It's also worth noting that there's probably no mix of fighter and herald levels that
won't be fun to play. The two classes compliment each other well, and the abilities mean that while there are trade-offs, you probably won't hamstring yourself, either.