Yes, I can think of several (not least of which is I have dealt with banks where those transactions can take over 30 days to process); however they would be all over the place speculation wise and even then some people may not “accept” some of the reasons I could come up with as each person has their own limits as to what is reasonable.
That said, multiple months of waiting sounds unreasonable without any context; and is not a time period I personally would wait without considerable documented communication with the company and the payment processor.
There has been considerable discourse on the forums regarding refunds and, taking posters at their word, some people have gotten refunds quickly when communicating with Purism, some have involved legal threats and believe that has gotten their refund more promptly, and some have done charge acts through their financial institution. There does appear to be inconsistency with refund payments.
One refund situation I did not see a resolution for was a user that claimed to have made payment via bitcoin, and taking them at their word there was lacking communication from purism on that front.
I’m not claiming that any of the non-shady reasons I can come up with are relevant, just that they exist and each individual situation is different and trying to apply a blanket timeline to all of them is unreasonable. That general timeliness that fits the majority of situations is a good reference point for when to reach out to purism (or any other vendor) to discuss the specifics of your situation and whether or not you can reach an amicable solution without involving other parties, but it should be just that, a guideline not a hard and fast rule.