Why is St Arsenius (Matsievich) "dubious?" In what sense?
He was born into an Eastern Catholic family, but became Orthodox later.
A very devout Christian and Bishop, he was involved in the glorification process of St Dimitri (Tuptalo) Metropolitan of Rostov and proomoted his veneration throughout his life.
He got into trouble when he opposed the Tsarist Russian government's attempts to control the Church via the Synod and was condemned for this under Catherine, stripped of his offices and sent into exile to be walled up in prison.
The Russian authorities who held him were ordered not to tell the local Orthodox populace who their prisoner was. But they came to know him as he communicated with passersby who later brought him food through the prison window.
St Arsenius was treated shamefully and cruelly - there were no washroom facilities in his cell and he was barbarously underfed.
When he shouted for help and for justice, the prison guards came and stuck a wooden stick into his mouth to get him to be quiet.
Finally, the Metropolitan reposed after enduring terrible suffering. A guard was sent to look in on him (he too did not know that Arsenius was an Orthodox Metropolitan, but only the prison warden). As he looked through the peep-hole, the guard saw the entire prison cell lit up with a strange bright light with an Orthodox Metropolitan standing there arrayed in beautiful robes! He went yelling to the warden that there was a Metropolitan in the cell and how did he get in there?!
The warden came to see what had happened and found St Arsenius lying dead.
More than two hundred miracles through St Arsenius' intercession were formally recorded by the Orthodox Church and he was slated for formal glorification in 1918, but the Russian Revolution prevented that. He was, instead, glorified in the year 2000.
His local veneration began almost immediately after news of what happened to him got out. Russian aristocrats would pay him tribute by building small replicas of the walled-up prison in which the Great-Martyr suffered in their fields and gardens.
Holy Great Hieromartyr Arsenius, Metropolitan of Rostov, pray unto God for us!
We know St Procopius of Ustiug was a former Roman Catholic due to the fact that he prayed his Latin psalter throughout his life (which was found on him when he reposed) and then through the witness of others who knew him. He was a Germand merchant who came to Ustiug and who became an Orthodox Christian and fool for Christ's Sake.
Dressed only in a light shirt, he wandered begging for alms and praying. During one severely cold winter, it was so frigid that birds were freezing on rooftops and were falling down dead . . .
No one would receive St Procopius into their warm homes due to his appearance. To find some relief from the bitter cold, he tried to join a pack of dogs who were huddling together - but even these ran away from him.
St Procopius then thanked God that he was rejected even by the dogs in the streets. He then lied down in a corner and made the Sign of the Cross asking God to receive him into His Kingdom as he was freezing to death.
Later that evening, the people of Ustiug saw a wondrous light coming from the Church (where the miraculous icon of the Ustiug Mother of God of the Annunciation was venerated).
Many ventured forth to see what was going on. When they came into the Church, there was St Procopius standing in prayer near the iconostasis, perfectly alive and warm, with holy and miraculous oil dripping from his outstretched hands!
I think that what all this means is that when you Orthodox get a Catholic convert - you are getting the very best!

Alex