The game that devolves into 90% theorycrafting, 10% hitting two buttons?
That's hardly a point in the idea's favor. Nor an MMO, for that matter.
I suppose we could say PoE's sphere-grid-like system does a... kind of decent job(?) of exploring its world's undermechanics and finding crafty and/or practical solutions through that system, but in terms of gameplay it's just a weirdly convoluted system of limitation. Rather than having many abilities to play around with, the way builds work in that game funnels you into a very narrow skill focus and rather few near-optimal choices. Only a sliver of its complexity even affects gameplay; the rest is "menu-play".
Let's take another game with similarly few action choices, Overwatch. To be more fair, let's also take the least FPS-like DPS we can from that game, Doomfist. He has only 3 abilities and an ultimate. But because the design of each of those abilities lends each deliberately different uses, be it escape, setup positioning, punishing, displacing, or pure damage, the character has many ways he can be played -- the "Batman" assassin (solo-kill DPS), the vanguard (psuedo maintank), the enforcer (psuedo offtank), the besieger (initiator), etc., etc. -- even if one manner might be more effective than others in a given match-up. Now, that last bit is always going to happen -- there will always be a more effective solution -- which is why it's fun to learn or to gamble on which strategy will be the most effective. The difference is, Doomfist can be designed to have more flexibility in his kit, which is the opposite of the goal of min-maxing in anything like PoE, where all capacity should be focused on a very particular goal, usually pure damage simultaneous with some manner of soft CC to ease mass slaughter. Heavily system/menu-derived games like PoE don't allow the player to change how they play so much as solely what they play. If you want to change with the circumstances, you must leave combat, possible even go to town, possibly even spend a bunch of gold, and change how you're allowed to play through a menu.
Now, do I think customization can work? Yes, but they must be designed...And that is a very tall order (though points 1 and 2 really should happen regardless), to the point that unless the system is highly lucrative in terms of character-building, world-building, and player-world interactions, I would sooner recommend just making flexible classes/jobs without customization.
- in a game with numerous and balanced undermechanics to allow for a diverse desirable effects and flexibility in how skills are used,
- enough short-term gameplay goals (via things like Stagger or suppression by damage, etc), and enough long-term tension (via things like Enrage) to make both burst and sustained outputs feel powerful and necessary,
- with a priority on the gameplay they provide, rather than merely being a sandbox on which to build a higher-throughput build than the next guy until he copies your build anyways, and therefore a relatively tight balance between choices to allow for many builds within close performance of each other in the context of a given fight, and changes in fights to bring new builds into the lead without obliging them, and
- via a system in which players are not obliged to spend time in menu, even if that means denying them optimal choices at certain times (e.g. they must build their spec around the entire raid, rather than on a per-boss or per-hallway basis).