If wizards can be balanced, why can't warlords?
Wizards are balanced?
Seriously, though, 'balance' just isn't the concern in 5e it may have been in prior eds. A 5e class dropped into 4e would be horribly broken, because 4e class balance was very robust, facilitated by a common structure, which those classes violate in major ways. A 4e character might top out at 4e daily attack powers at 20th, while a 5e caster typically starts with 3, and gains more ever single level. Neither scale like they did in 3e and earlier, but the 5e caster is also wildly more versatile - the 4e character picked those 4 dailies one at a time as he leveled, and could retrain one each level if he wanted to, between levels, they stayed the same, and he could use each one exactly once, while the 5e caster can change prepared spells every day, and choose which to cast with a slot round by round, potentially 'spamming' whatever daily spell works best in the situation. Then there's 5e classes that have no dailies and even less flexibility than 4e characters did.
There's no way the same kind of balance applies in 5e. Not even close. 5e balance is more fluid and under the control of the DM, coming mainly from the way the DM presents challenges - 6-8 encounters with 2 short rests between long rests puts enough pressure on spell slots (and other limited-use resources) to keep casters from applying maximum spell power to every situation, and from there it's just a matter of presenting a variety of challenges such that each character gets to step in with the right spell, or ability, or skill check, or step up and do enough DPR, to have his moment to shine.
The Warlord, by the very nature of the concept, can't be as versatile and flexible as a caster, simply because even the wildest narrative constructs it's ever used don't come close to doing the range of things magic can do in D&D, nor are those things the warlord can do (mostly exhort allies to do) unlimited in nature (you can only dig so deep so often), conversely, a faithful implementation be as narrowly DRP-focused as existing martial classes. That puts the warlord squarely in the center of the pack as far as the difficulty of balancing it goes. A DM who can handle balance for a game that includes a Champion, Cleric and a Wizard would barely notice the challenge of balancing a party that included a Warlord.
A 4e warlord powers, directly ported to 5e (changing free action to reaction and surge to hit dice), would be underpowered. You simply don't have enough actions, and int isn't high enough for game breaking bonuses.
A few minor-action attack or multiple attack powers or a free-action chain combo and you could 'break' (or merely optimize, sometimes there's not much of a difference) an early 4e striker pretty dramatically. The infamous Fey-Charger, for instance. The worst of those - which, I guess, you could assume is were they drew the line between 'broken' and 'optimized' - would get 'updated' out of existence fairly quickly. Warlord attack-granting could figure into some of those builds, particularly whole-party-optimization builds, because it was, afterall, a force-multiplier. But there was nothing innately broken about the Warlord's attack-granting, because the action economy was fairly tight, and attack-grants reflected that. The action economy in 5e is even tighter, with no minor actions, and with all off-turn actions consolidated into a single reaction. There's just not a lot of room to over-optimize attack granting. Summoning, for instance, is already 'more broken' than anything that's been proposed for the 5e warlord.
