All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The Sahaba(Companions) are the best of the Muslim Ummah after its Prophets(PBUT), and all of them are upright and trustworthy. Therefore, it isn`t permissible to impeach or belittle them for they are, indeed, the best of the best. Allah, The Almighty, said, "And the first to lead the way, of the Muhajirin and the Ansar, and those who followed them in goodness - Allah is well pleased with them and they are well pleased with Him, and He hath made ready for them Gardens underneath which rivers flow, wherein they will abide for ever. That is the supreme triumph."{At-Tawbah, 100}. In fact, saying "May Allah be pleased with him', "May Allah forgive him" or "May Allah have mercy upon him" is a prayer included within righteous deeds. In addition, it is desirable to ask Allah to forgive the dead of the Muslim Ummah, and since the Companions are the best of peoples evolved for mankind and the best of creatures after the Prophets(PBUT), so they are more deserving of this supplication than others.
An-Nawawi(May Allah have mercy upon him) said in Al-Majmoo’,{6/172}: “It is desirable to ask Allah to be pleased with and to have mercy upon the Companions and the Taabi’is (Successors) and those who came after them - especially the scholars, devout worshippers and all other righteous people. One may say "May Allah be pleased with him" or "May Allah have mercy upon him" or the like. However, when a particular Companion is mentioned before a person, and that person doesn`t say "May Allah be pleased with him" with no intention of belittlement, then he/she is sinless.
Nonetheless, mentioning a Companion without saying "May Allah be pleased with him" indicates incomplete reverence, and it is undesirable because complete reverence is to say "May Allah be pleased with him" every time he is mentioned, even if no disrespect was intended. But, not saying "May Allah be pleased with him" out of disrespect, is prohibited since it is impermissible to belittle any Muslim, let alone the Companions whom Allah has chosen for the best of His creatures, Prophet Mohammad(PBUH). It is better for a Muslim to busy himself/herself with pleasing Allah, and leave out matters that aren`t beneficial to his faith or life. And Allah knows best.