If B&M and Merlin put in the same amount of effort as Nemesis then it would be received well... Enthusiasts might well be fickle, but do also tend to appreciate effort being made to produce a quality ride based upon the park's likely budget (hence why Lost Kingdom at Paultons gained plaudits, as the £6 million budget looked like it had been used thoughtfully)...
Swarm has a good story? No, that's just silly... Aliens are attacking the park and we're escaping them by riding them? And that's if a story actually exists of course (like Thirteen's 'story'), not that a ride needs a story to be good of course, but Swarm's never exactly been rated for a high quality story... The queue videos are cool and unique, unfortunately good queue design has seemingly been forgotten in theme park recently (not just at Merlin parks, but Jesus that hidden cattlepen can get in the bin)...
It's also not even the best Merlin Wing Rider either (that's Raptor), whilst the props and effects are good (when working), there's still issues in regards to having more than just a few props stood in random positions, it needs to all tie together in a logical sense, and adding a giant billboard doesn't help matters, and nor does the missed opportunity that is the station (seriously, that station should've been indoors with fantastic lighting and mood, plus could still have the ride 'crashing' through the building anyway, it's my biggest bug bear with it)...
I think there in lies Merlin's main flaw, there are good ideas in these attractions (unless it's IAC), but things never stretch far enough to make it a complete addition, there's always one or two often basic issues they've got wrong in the design process that prevents a ride from being of highly rated... I'd probably say Raptor is my favourite overall Merlin addition because it is the closest thing to combining a thought out attraction, theme and atmosphere together, it looks like they made an effort because it was the new icon ride for the park, and for me, it just works well (until Merlin decided that Gardaland would sell a year long unlimited Fastrack to MAP holders, morons)...
I'd say people are noticing the issues more and more if those middling reviews on Trip Advisor are anything to go by, if things keep dropping in terms of maintenance and such, people do notice and are likely to mention it...