To start, have a direction to follow (What are you trying to show?). From beginning to end, can one random individual audience member grasp what you're trying to show without you needing to explain your concept outside of the amv?
If yes
If no (I'll need to make a flow-chart full of if-else statements)
Personally, I think that there is a spectrum on the difficulty of achieving flow in an amv. I believe that difficulty of achieving flow depends on (i) source material and (ii) personal style of editing.
(i) Source Material may make or break an amv from the start. I think there are two factors to consider in this scenario: (1) Ease of matching video with audio and (2) Amount of Material being used
(1) Ease of Matching Video with Audio: Basically, two examples of easy matching is "Kimi Ni Todoke" with "[Insert Slow Love Song]", or "Dragon Ball Z" with "Linkin Park". They just match and they're easy to edit because the video material is attuned with "Love" or "Action". Hell, this is probably why Shelter had such amazing flow (My dream ;-; ).
Now... If you attempt to match Kimi Ni Todoke (Slow Romance Anime) with Linkin Park, you will most likely spend a lot of time trying to match the two sources together because Kimi Ni Todoke is NOT an action anime. Note: This is not implying that matching the two source material is impossible, but difficulty rises when they are not compatible from the beginning. Fluffy Kittens and Rainbows and Death Romance prove that this is completely possible but, the larger the mismatch, the more creative (and fun ) you have to be with flowing the materials together.
(2) Amount of Material Being Used: Basically, the more material you plan on using in one sequence of flow (which can be from 3 frames to, I'd say, approximately 4 seconds), the more time-consuming the amv becomes. Umika (RIP) is probably my example for that.
(ii) Personal Style of Editing just dictates whether you have fun editing the material or not. If editing the material is easy for you, you probably find that particular style of editing fun (i.e. fast, action paced verses Slow, Romance based; Multiple Anime Sources verses One Anime Source). Basically, Have fun with what you do. Sometimes you're better at editing slow videos than fast videos. Other times, it's the other way around. Who knows