I haven't played the game, so cannot confirm, but considering what I have saw in youtube videos, the game is more similar to Melee than 4.
And I consider Melee the hardest one.

It was a little difficult at times for me but nothing took more than a couple of retries. Some of them actually handed the match to me like Captain Falcon who turned round and leapt to his doom right at the start or K.Rool who got take out by a Klaptrap within a few seconds.

That said, I don't think unlocking characters was a good idea, I think fighting games should present the full roster up front. There is more than enough to unlock in terms of spirits, music, items, challenges etc that it would still offer plenty for solo play and when the real meat of the game rests in multiplayer it is just frustrating that almost every fighter needs to be unlocked. I'm now really enjoying the game but it was a frustrating grind for my first day of playing just so I could start to enjoy the game.

I got into the series with Melee. If I can get over that, newcomers can surely get over this.

I've bought Smash Bros. games in the past and never spent much time with them. I've started this one and I feel that it's better geared towards a single player, but I barely know the key presses and I've lost most of my unlock battles. That's okay though ... the game would be stupidly easy if someone who knew nothing could just waltz through the game. I imagine that this game is ludicrously easy for an expert and a worthwhile challenge for a keen newcomer.

FML WHAT ITS YOUR ADDRESS I'LL SEND YOU A PARTICIPATION AWARD.

Loosing a few games is normal. I would be concerned if I couldn't loose in a game.

The game is actually very casual friendly. I have had three people who never play Smash play it and win against the CPU. In classic mode the CPU automatically adjusts to your ability if you lose.

Casual play isn't new players, or retarded/poor skilled players, it's about players who have more of an intermittent play schedule (a few minutes at a time, usually one or more times per day) and don't sit down and binge for hours.

Now im even more hyped to play it.

So far I haven't had much of a struggle in WoL on normal, though I've only covered probably 30% of the map. Kirby's one of my best fighters (maybe THE best) so I usually just wail on enemies with him.

I simply maxed out a couple of spirit cards to 99 from each category (attack, grab, defense) and equip whichever gives me an advantage in a fight, tack on some augmented spirits to the cards (forget what those secondary spirits are called), and I'm good to go. Funny enough, the only times I've died multiple times to my recollection have been fluke items or environmental hazards like getting zapped into oblivion in the Pilot Wings stage or being rocketed up into the sky before I can avoid the item.

Classic Mode has been pretty simple for the most part though I start to run into problems once the intensity crosses the 7 mark.

Have unlocked like 20 fighters so far. Beat them all in one attempt, though I've had a few very close calls.