Last week, I did 3 hikes that if you just looked at the top line numbers, you'd say they are 3 very similar hikes:

Flume and Liberty: 8.8 miles, 3677 feet. (9.2, 3688 by GPS, 1.0 MPH pace)
Garfield: 9.5 miles, 3079 feet. (9.7, 3061 by GPS, 1.4 MPH pace)
Tom, Field, and Willey: 8.9 miles, 3583 feet. (9.6, 3615 by GPS, 1.2 MPH pace)

All 3 hikes are about the same distance and elevation.  Maybe you can say Garfield would be slightly easier since it is about 600 feet less elevation, but you wouldn't really say Flume/Liberty would be that different than Tom/Field/Willey, they're within 100 feet of elevation and 0.1 miles by map.  You couldn't be any more wrong.  Subjectively, Flume and Liberty are significantly more difficult than the other 2 hikes.  You could argue over which of the other 2 was more difficult, they both had their challenging sections, but I would call them similar with Tom, Field, and Willey probably being a hair harder.  Based on the pace of the hikes, Garfield would be the easiest.

So, the top line numbers are only part of the story.  Without the advantage of hindsight, how could we know this going in?  Well, there are a two main ways.  The first way is to look at the elevation profiles of the trips, and specifically the slope of the profile:

Flume and Liberty

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Garfield

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Tom, Field, and Willey

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These are 3 completely different profiles.  Flume and Liberty has a significant elevation climb in the half mile before Flume.  The map planner places it at over 90%, but the GPS trace only has it in the 50-60% range.  Garfield on the other hand has only a short, 0.2 miles of steep right at the top, and Tom, Field and Willey (with Avalon added on) has a couple short steep ups and downs spread across the trail.  The slope would tell you that Flume and Liberty would be more difficult, possibly going into rock climbing which is indeed the case.

The other way is to read the trail guides published by the local mountain club.  In this case it's the "White Mountain Guide" by the Appalachian Mountain Club (AMC).  Reading the guide you get for the Flume Slide Trail (our way up Flume): 

"The trail's upper section is extremely steep and rough, with smooth, polished rock slabs that are very slippery when wet (and they are nearly always wet due to the many seep springs on these steep slopes).  This is one of the most difficult sections of trail in the White Mountains; several serious injuries have occurred here.  Flume Slide Trail is not recommended for descent, and its use is discouraged in wet weather, when the ledges are more dangerous than usual." (31st edition, page 165)

For the Garfield Trail:

"Most of the way the trail follows an old tractor road used for access to the former fire lookout, and its grades are easy to moderate all the way to Mt. Garfield, except for the short, steep pitch on Garfield Ridge Grail just below the summit." (31st edition, page 176)

For Tom, Field, Willey, and Avalon:

A-Z trail: "A-Z trail descends slightly, crosses a brook, swings right, and soon begins to climb more steeply.  The trail bears left and continues climbing steadily reaching the hight-of-land at 1.0 mi., where Mt. Tom Spur diverges right." (31st edition, page 189)

Willey Ridge Trail: (we did this both directions): "It then climbs, steeply at times, along the west side of the ridge, passing a blowdown patch that offers a restricted view into the Pemigewasset Wilderness." (31st edition, pages 192-193)

Avalon Trail (we did this down): "Avalon Trail soon begins a sustained climb that is steep, very rough, and rocky.  Near the top, the grade moderates and the footing improves, and at 1.8 mi., in the small col just below Mt. Avalon's summit, a 100-yd. side path diverges left.  This path climbs steeply with several ledge scrambles to this fine, ledgy viewpoint; ... Avalon Trail passes through a flat, ledgy area with restricted views, climbs easily through a  switchback, and then swings right for a steep, rough climb, with some slippery ledges and one restricted view to the northeast." (31st edition page 189)

Doing the research, both the book and looking at the elevation profiles and slope, both tell you that by far, the Flume/Liberty hike was going to be the most difficult, which is why we did it first with fresh legs.  It would then say Tom/Field/Willey/Avalon would be next most difficult and Garfield would be the easiest.  This is what my experience on these 3 hikes is too.

Now why did I not look at crowd sourced information like AllTrails?  It's because it's crowd sourced.  A person's fitness, hiking skills, hiking history, and personal fears and phobias all show up in the reviews.  You have no idea what their experience is compared to yours, or how they hike compared to you.  Unless you've hiked with someone in the past, you have zero basis for comparison.  Overall, I personally found that the flume slide section was one of the harder sections of trail I have ever hiked.  It ranks up there with some sections in the Adirondacks (Sadleback Cliffs, Little Haystack descent to Haystack, Slabs on Algonquin trail, Allen Slide).

I know I said that the hike was probably in the top 5 hardest hikes I've done, and I think that is probably true for hikes I've done since I started the 46 in 2020.  That said, that first hike up Giant in May 2020, and a hike I took a friend on up Algonquin a few years before that were the hardest hikes I've done to that point.... After doing them a couple more times, they don't rank up there because I am in much better shape and have significantly more experience.  

Crowd sourced reviews may give you an idea, but they are not always reliable or relatable to your personal hiking abilities.  This is why objective information like the elevation profile and slope combined with mountain club trail guides that are focused on just describing the trail and not giving opinions on it are much more reliable.