Is Josephus account of destruction of Temple considered reliable?

There are parts that are reliable and parts that are not so much. You can readily tell, simply by the name of the work itself, that it is written from the Roman standpoint (to the Jewish people, this book would be entitled 'The Roman War' after all) and it's skewed to a pro-Roman stance from that. That being said, he appears to be a mix bag himself with his position lying somewhere between loyalty/appeas*m*nt to the Romans, but somewhat faithful to the Jewish aristocratic's he descended from (stating the aristocrats were peaceful and accomodating) instead blaming the conflict on 'Jewish brigands/rebels/bandits' (probably 'extemists' in todays language)

He does appear to have access to military documentation from the Romans side and much of this information is verifiable. He also got distances very close to exact.

http://www.centuryone.com/josephus.html

Some of his facts have turned out to be quite accurate, especially ones that we can directly fact check such as geography: The most obvious data for examination, it would seem to us, is archaeological material. In many instances, numerous details provided by Josephus can be checked, including architechural data, and their accuracy confirmed. Such precision, where it can be established, is surprising, especially since the information was set down in writing years after Josephus had left Palestine. In addition, it is clear that in some cases he is describing objects that he cannot possibly have seen, let alone measured. Thus he probably never visited Masada or set foot on its summit, so he cannot himself have measured its walls. For sixty years preceding the Great Revolt, the desert fortress was occupied by a Roman garrison and civilians were not normally allowed entry. Even so, he writes in War (VII, 286) that the walls of Masada were seven stadia, i.e., about 1300m. long.1 And so indeed they were.2 Similarly, he describes in War (I, 403) the walls of Samaria-Sebaste, built by Herod, as being twenty stadia long (3720m.). This figure also approximates to their length as unearthed.3

Whether he was there or simply had access to someone else's numbers is debated in a few contexts, but his numbers were accurate for the most case.

As for the other portions...you find a mixed bag of internet debate on the topic. He is widely agreed upon as exaggerating and story telling, though it's not easy to tell how much of his story is fiction vs non-fiction, in good part due to their not being other works to readily compare him to.

Oddly enough I can find a few references that suggest he was a Jewish apologist struggling between pleasing his Roman overlords and while trying to display Judiasm in a positive light

https://answersingenesis.org/bible-history/is-josephus-reliable/

His pro-Jewish sympathies emerge clearly in The Jewish War. That work portrays most Jews as peace-loving citizens. He blames Jewish zealots, whom he calls “bandits” and “brigands,” for the collapse of Jewish society in the first century. Antiquities, written later, attempts to show the superiority and antiquity of the Jewish culture. To achieve that end, it tends to exaggerate the good qualities and ignore the unflattering failures, such as Aaron’s golden calf, in an effort to promote the Jewish cause.

One of my preferred writings on the topic is here by Eric D. Huntsman: https://scholarsarchive.byu.edu/cgi/viewcontent.cgi?article=3183&context=byusq

it was Josephuss internal bias however that had the greatest affect on his selection and use of evidence sometimes this bias was purely personal such as when he exaggerated his own achievements and skills or tried to justify his surrender at Jotapata more importantly. Josephuss subject presented him with two seemingly conflicting loyalties he was at the same time pro roman and pro jewish. His solution to this dilemma was to blame the war on neither the romans nor the aristocratic jewish leaders whom he regularly portrayed as desiring peace and working for accommodation modation instead he held responsible the jewish extremists whom whether they were the zealots zealous in jerusalem or the sicarii sicardi who seized control of masada 33 he called lestesii lestsiste or bandits this shifting of blame however is probably only the proximate purpose of jewish war Josephuss ultimate intent seems to lieliefardeeper far deeper 34 his later works especially antiquities and against aaion apion were written largely to defend the history and current rights of the jews outwardly the jewish war by shifting the responsibility of the war away from the body of the jewish people achieved this same purpose inwardly however josephuss jewish war served to promote within the jewish community greater openness and more cooperation with rome 35 the roman empire with divine sanction had conquered the jewish homeland it was necessary therefore for the remnants of the jewish people to submit to gods will and work within the roman system to preserve their way oflife

(copy and paste above didn't go to well...weird formatting)

Ultimately the answer to your question is we don't know how accurate he is being here as there is little to nothing to compare these accounts with. We can verify a bunch of his geographic data as accurate, but at the same time find many examples of extreme exaggeration and quoted speeches that he could not have been around to hear. This is a trait of the majority of Historians from this time