The Queen has made comments about Scottish independence and other political things. Charles has his spider letters. Charles has undertaken similar legal action as H&M including against his diary being published. Several royals have edited magazines and newspapers, only Meghan was criticised. When Charles and William have visited an Islamic centre where a/some terrorist/s visited, nothing was mentioned... but when Meghan had a charity food project based in their kitchen, the headlines screamed about the 'links'. None of the women volunteering there were linked and were totally innocent.
There's hundreds of examples like this, either the other royal is not criticised or they have been previously, but it is never mentioned again/forgotten about
This post sums up the ridiculousness of this debate, to me, which is the confirmation bias of many taking part. You mention the Queen's comments before the Scottish Independence referendum, Charles' black spider memos - these are excellent examples of times when members of the royal family have been roundly criticised by the press for their actions.Perhaps that is why they are memorable? Other members of the press get criticised all the time. Do a google search for "Prince Charles criticised", "Prince William criticised" etc and a long long list of articles pop up , showing they have recently been criticised for taking private jets, going grouse shooting, their friendships, their holidays, their spending - any number of things. That's just the most recent stories. Charles was also criticised for his views on organic farming, modern architecture, shaking hands with Robert Mugabe at Pope John Paul II's funeral - I could go on (and on and on - he also opened Finsbury Park Mosque, stronghold of Abu Hamza - that and the terrorist links have definitely been brought up). Some of it inevitably falls out of people's memories as time passes, which is the nature of human recall, but it all happened and continues to happen. It gets discounted by those of the view that only Meghan gets criticised, though, because it doesn't fit in with their view of the world. And as for the forgotten about - they would have been in time too, save for the really memorable ones I imagine, just as has happened for other members of the Royal Family, had Harry and Meghan worked for a number of years and risen out the surge of press attention that they were always going to get as the new stars of the show.
A lot of the thing that people get so exercised about in Meghan's case also isn't criticism, it's just clickbait using her name to attract attention. The avocado story, which gets referenced again and again, had an attention grabbing headline: "How Meghan's favourite avocado snack - beloved of all millennials - is fuelling human rights abuses, drought and murder". But it's not criticising Meghan! It is using her name to attract people to open the article , while explicitly pointing out avocados are also beloved by all millennials - i.e. she is one of literal millions when it comes to liking avocados. Pointing at that as an example of Meghan being bullied by the press is ludicrous to my mind, as it is in fact rather a reflection of the huge amount of interest there is in her out there, and in people clicking to read stories to which she is tenuously linked. Which, given we know she is paying a PR agency to promote herself, actually sounds like the kind of position she wants to be in. No-one is shoehorning nonentities into stories to get people to read them, after all, they do that with really famous people....