Its funny because if it was a tv series with two male characters just being "buddies" with the odd joke there probably wouldn't be the negative response.
What is bad (IMO) is that Ruth and Tosh sit round having cups of tea and biscuits, and poor old Sandy is now doing all the work because Billie seems to now be a home body.
And then when Sandy gives Ruth and Tosh the relevent info they go off and make a mess of it.
I think Tosh's back story has been weak for some time, and we just get her being tired but luckily her home husband is keeping everything going.
And Ruth, as commented earlier, it was so obvious a new love interest had been magicked up from somewhere, with all the credentials of knowing every one she knows, but somehow they dont (as yet) know each other.
I think the best bits of Duncan and Perez where the shared parenting bits.
On one level apart from criminals, Tosh and Ruth dont have much in common. Or a common bond.