He was in direct command of the Battle of Antioch with the Persians,which he lost.He also appointed the commanders who fought at Yarmouk.My opinion of him is that he is capable of doing brilliant things but at the same time,he is also capable of fucking up in the worst possible way.This is why I think he's overrated.
I'm sorry, but I must disagree with you on this.
Heraclus was quite easily the greatest ruler and General of the Byzantine throne.
To start with, Maurice an effective general and ruler and still one of the best Byzantine rulers, mainly for his conquest of Iran and the throning of Khisrau as the Shah of the Sassanids and his actions in keeping the Avars and remants of the Hunnic horde at bay. Was overthrown by the seemingly competent (I don't doubt he was) Phocas, who then took the throne murdering Maurice and his family.
This sparked the final Sassanid- Byzantine confrontation with Khisrau as a sign of admiration took up arms to avenge his former patron Maurice and find an heir to the throne and place him on the throne of Byzantium, as Maurice had done for him.
Khisrau then cut through the armies of Phocas as he was inspired it mentions and conquered much of the Mid East. Further he signed an alliance of sorts with the Avars which had broke the lines in the Balkans following the mutiny of Phocas and with their various forces including Slavic tribes marched southward.
However Heraclus who then rebelled from Africa, took the throne from Phocas (no small feat) and killed him in Constantinople, was at the helm of the Byzantine war effort. Despite all the odds against him with the Avars and Sassanids on either side he defeated both armies in battle and invaded Iraq taking much land forcing the Sassanids who had just previously ruled the entire Mid East to capitulate and surrender under any and all terms. Mind you this was not against a feeble ruler but Khisrau, a famed general for the time period. No other Byzantine tuner achieved such a great feat as this, none.... To turn a war with such imbalance around on both sides competent outclassed two of the most major powers in the classical world and gave the most resounding victory Byzantium would ever give to an Easter for ending intermittent wars of over 400 years. Basil II did not do this much that is assured.
In terms of the Arab conquest, look at the Sassanids.... They were completely torn apart and emasculated by a highly motivated and large fighting force. Compare that to the Byzantine situation....
The first of these battles that Heraclus engaged in was the skirmish at Mu'tah where Heraclus and the Ghassanids massacred the Arab army led by Khalid ibn Walid and Thabit ibn al-Arqam. This mind you was likely a massively decisive victory as Mummad himself shied away from further moves north into Byzantium for fear of losing an even larger army, and since no formal war was declared by Heraclus, he felt safe.
After this battle, 629, Heraclus had fallen ill and at the worst time. As the Muslim forces fresh off of constant war in Arabia invaded Syria in earnest.
The first major engagement at Ajnadayn was an enormous victory for Khalid Ibn Walid and his army made primarily of formerly pagan Makkans, with a severe loss of Byzantine general Vardan. The losses are reported as being heavy and the death of many nobles of Makkah were killed which solidified the power of the newly empowered Ansar (helpers of Muhammad, the ones closest to him).
Then shortly after the Arabs won in Fahl defeating Theodore with a combined army of Khalid ibn Walid and Amr ibn al-'As.
Then culminating to the battle of Yarmouk where the larger Byzantine force was defeated by Khalid ibn Walid. This however was genius by him, as he is noted to have concentrated on a decisive battle, exactly the opposite of the Byzantine general strategy of fortification and skirmishes. Thus in a similar manner to the Russo-Japanese, the glass canon strategy worked to perfection as the smaller more determined and experienced army ravaged the large exhausted army.
Never before had the Byzantines faced such a tactic and where used to the style of warfare waged before of fortifying everything and waging large skirmishes across the field and further the Byzantines were even less prepared to fight Arab armies of mass light archers determined on decisive battle victories. Previously they tasked the now mutinied Ghassanids to keep the Arabs in check.
One then must understand that Heraclus was faced whilst a sick man, an opponent of extreme power who due to distance was near destined to capture lands as how where the Byzantines to invade Arabia? They would get lost or swarmed in the desert. Further with the complete collapse of the Sassanids, they had no possibility of a two front war. Then we must realize the victories of Heraclus in stopping the Muslim invasion into Anatolia and stopping the flood of the Muslim armies as the Sassanids couldn't.
Further do not forget the even more relentless invasions as launched by Muawiyyah and the Umayyads under the concept of Baqqiyah wa Tattamadad, to wage war with no end, the peace is never achieved until Rum is within Islam and at the time many Muslim believed the world after which would end. One cannot describe the outstanding abilities of the Byzantine empire in warding off the Umayyads paving the way for Basil II, easily in my opinion the most impressive period in terms of Byzantine military achievement.
Yet, after all the loses Byzantium was dealt, they outlived both the Rashidun, Umayyad and Abbasid. It was because of Heraclus and his close successors for stabilizing the situation unlike the Sassanids which allowed themselves to be absolutely crushed. The Umayyad crumbled and the Abbasids were destroyed by internal contradictions. So who truly won the overall Arab-Byzantine war? As there was never a peace...