SLHG barely affects the top-tiers, as most of them are known for having safe approaches. The ones who benefit the most are Mid-high tiers. (Meta Knight, Greninja, Cpt. Falcon, Ike, Marth etc.)
The rest of the cast however, especially heavies, also have benefits.. Samus for example, gets access to safe aerials and a devastating combo game. Meta Knight no longer has to play patiently, he can play as aggressive as he wants. Marth can Short-hop F-air with little to no worry, and link together F-airs like it's nobody's business.
All in all, SLHG is a huge improvement to overall roster balance. It greatly boosts low-tiers, and makes every game about Skill-level, as oposed to just match-up knowledge (aka vanilla Sm4sh). Any character can win an SLHG match-up if you're good enough, some match-ups are doomed in Vanilla no matter how good you are.
SLHG favors Technical Ability (Tech-chases, fast-falls, footstools) over Discipline/Knowledge (Reading DI, Abusing Shield, Hard Reads), similar to Melee, PM, and 64.