Let me say one thing ahead: I'll do a lot of criticising here. That doesn't mean I don't like this mod - quite the opposite: I like it and wopuld like to see its weaknesses reduced and the strengths built upon, so dear design team - don't take it badly, it's meant in a good way.

So lets start out with the classless class system.
It's a neat Idea, that sadly doesn't (yet?) work half as well as it sounds.
While it's neat to be able to equip every Soldier the way I like, in the beginning the soldiers are all more or less alike and in the end when they're specialised by the perks chosen at levelups, the AWC comes in and gives them completely useless bonuses. My Sniper suddenly loved Shotguns for example. Also to to specialize the crew yourself, in some levels there are just no good picks.
My suggestion to change that: Either add even more Perks to choose from each level or ditch the classles system. Also please either make the player directly choose the AWC-Perk or make the game choose one that is in line with the soldiers chosen perks and/or history. A Soldier that never had another weapon in his fingers than a sniper rifle shouldn't suddenly find out how much he loves a shotgun.

I love the new Advent factions. I think it's a great Idea, and gives a great lot of variation to a (in vanilla) boring enemy. On the other hand it took me a huge load of time until even one real alien showed up (with the exception of sectoids which were around rather early).

I also like the diversity in weapons. Maybe there is a little balancing needed, because the Mag/Gauss Weapons should probably be the ones with the highest damage output, to compensate for the fact that they do neither pierce nor shed armor. Still, really good stuff here, especially the chance of looting alien weaponary.
What ties well into that is the immense Loot system including the option to reverse engineer all of it. I really, really enjoyed that.

Sparks are practically useless. Even with the immense health and armor the adonized chasis brings, the unability for it to take cover makes it the first and easy pray for enemy crits.

Speaking of crits - the enemies aim and damage stats are... poooh, off the chart. I know, this mod is meant to be hard, but I was really really pissed at the game quite a lot, even when playing on the easiest difficulty.
It's nice to have a challenge, but maybe tone it down somewhat on the lower difficulties, or at least on rookie.

Now to the ressource system. While it looked great at first and the idea is a good one, you run out of a lot of stuff really fast. I've already seen, this is going to be adressed in the future, which is great, but right now, it's really frustrating to have a s*itload of research options and not being able to do most of them due to missing ressources.

All in all Spectrum is lots of fun, even it's really hard, but it feels unfinished (maybe because it is...) and unpolished. I'm curious to see where this one evolves from here and will keep an eye out on it. Thanks for doing what you do here!

Cheers,
warchicken78