What's an appropriate phrase or idiom to describe something (e.g. a concept, tool, device, etc.) with many pros and cons. In fact, both parties have rational facts about their opinion and they are right.
a multifaceted issue vocabulary,com
complicated issues that have lots of contributing factors and generate more than two sides to an argument
“So, the issue is extremely multifaceted. Take any of these single issues and there will be a cascading effect in many unforeseen ways.”
An old but serviceable idiom for this might be:
"six of one, half-dozen of the other"
which is used to indicate a bevy of roughly-balancing pros and cons.
If "both parties have rational facts about their opinion" on a topic "and they are right", then the topic is, by definition, one on which reasonable people can disagree.
The essence of the phrase is that there are rational, factual arguments to be made favoring at least 2 different conclusions. It can be used when there is simply not enough evidence to support one conclusion over the other or when the conclusion is based on how you value the various pros and cons.
Reasonable people disagree about whether the first-move advantage in Chess is so big that a perfect player playing white (and therefore moving first) will always beat the other player. Eventually computers will be powerful enough to settle the question.
Reasonable people can disagree about the value of social media like Facebook, because on the one hand it appears to promote connection and maintaining relationships among friends, on the other hand there is evidence that on the whole it makes people more unhappy and displaces more meaningful in-person connection with friends.
I disagree with the choice of mixed bag because that phrase is about there being a variety of dissimilar components. It is true that when a solution is a mixed bag then often reasonable people can disagree whether the solution is on the whole an improvement over the original problem or not, but they are still different concepts.
For example, at this point in time, one can say that giving children a measles vaccination is a mixed bag, because there is some short-term discomfort and a very rare chance of some kind of long-term negative consequence to it, but it is no longer true that reasonable people can disagree about the value of it.
I also disagree with multifaceted as that indicates a topic is complex with many different aspects to consider. A topic (for example, the death penalty) can be fairly well represented by only 2 sides and still be one on which reasonable people can disagree.
I disagree with "six of one, half-dozen of the other" because that refers to a situation where 2 choices are roughly equivalent. In those cases there is generally no disagreement or argument, but rather both choices are widely acknowledged to be so similar that either one is acceptable and purely a matter of personal preference. Whether you put on socks and shoes in the order sock, shoe, sock, shoe or sock, sock, shoe, shoe is six of one, half-dozen of the other. There really is no significant rational argument to be made on either side.
Similarly I disagree with "on the one hand..., on the other hand...". What that does describe an situation where there are competing pros and cons, it generally expresses that the speaker cannot decide one way or the other because of that. Someone using that phrase would be unlikely to argue for one side or the other.
One hand, another hand: On one hand..., but on the other hand...
For example, "the tool is very useful and works best for what it was designed to do. On the other hand, it is expensive and has very limited use."
Note that the sentence did not need to start with "one hand," but we can still use the second or other hand anyway. You could convey the same intended meaning by using a word or phrase that sets up contrast by starting the second part of the idea indicating that what follows is in contrast to what came before, such as "the tool is unique and useful, but it is expensive and rarely used."