Its worth posting a copy of a letter signed by over 70 'rebels' - not all of them natural rebels either, that gives concisely the reasons they are against.
It is a bit difficult to read so I will 'paraphrase' the 3 key reasons for opposing this:
1/ It undermines the primacy of the Commons
2/ It creates hundreds more unaccountable elected politicians
3/ It would result in a 'less expert, less diverse and more expensive Chamber.