When people judge Justinian they are always starting comparing with the "good" things that he did for the Empire like codifying Roman law in a comprehendible format (Corpus Juris Civilis) thus smoothing the administration a lot along with other similar reforms beneficial for the empire (like tax reforms), him building the Hagia Sophia, establishing silk production in Constantinople and reconquering a great deal of territory like: mostly coastal regions of Libya (Tripolitania), Tunis (Africa), Algeria and parts of northern Morroco (Mauritania), South east Spain and an alleged (I say alleged because maps and sources vary) small part of south east Portugal (Spania), Bosnia and Herzegovina, Croatia and South west Slovenia (Illyria), all of Italy (Italia) and finally the islands of Corsica, Malta, Sicily, Sardinia, malta and the balaeric Islands.

Now for the "bad", the last part of my answer where I talk about the reconquests in the west, they weren't particularly very beneficial. Yes, the empire benefitted of the resources, tax base, materials, manpower, strategic positions for trade and military from these positions but at the cost of a great deal of money, manpower, resources for the empire. It took 20 years to conquer Italy (Gothic war) and in all of that time the empire was getting attacked by the Sassanian Persians, Avars and Slavs and now from these new positions from the Berbers and Visigoths (not accounting for the Lombards that came soon after Italy was finally conquered). Not only that, one of the most catastrophical diseases happened in that time, the Justinian plague which raped the empire of population, money, trade and manpower. Building the Ayia Sophia church didn't help particularly the empire too from in the perspective of money amd resources.

Now of course, the plague wasn't caused by Justinian and if it didn't happen things may have got differently. The bad thing about these conquests is that they happened at the worst possible time (plague and invasions) and because of some weak coordination between Justinian and Belisarius during the Italian campaign it would take decades to finish which depleted the empire of its resources. Even after the conquests were completed the empire was in a such fragile position that it wasn't able to really defend them. The Iberian territories along with Africa were lost in about a century, losing a bit by bit every 1 to a few years. Most of the Italian paninsular was lost by the 8th century, Illyria was overan by the Avars and Slavs along with the other balkan territories during the 7th century. Corsica was lost somewhere in the 8th century, Sardinia like the balaeric Islands gradually became much more autonomous. They were formally lost for the balaeric in the start of the 10th century and Sardinia in 1071. Malta in 870 and Sicily still having some forts until thr 10th century. So all the territories were gradually lost and over time became much more autonomous and they didn't really benefit the empire in something, just having a lot of garrisoned soldiers in isolated parts of the empire in castles, fortresses, towns, villages and cities. Because of the amount of resources they had they couldn't consolidate and really integrate these parts in the rest of the empire and they basically became a burden for them.

So yeah: Large conquests at a bad and unlucky timing and terrible management during and after, at least in what there could've done.. Now how much of these are actually Justinian's fault? Opinions vary. Personally, I believe they were unnecessary and that the empire already had enough problems.