Mostly this is just @Matthew's answer in different wording (I'm late to this question sorry).
So, if I can run all the applications of normal Ubuntu on Lubuntu, what am I missing with the lighter desktop environment? If the LXDE provides the same benefits as Unity does but using much lower RAM, why use unity at all?
toolkits or resources used (especially ram)
LXDE was very light, but it was also GTK2 (ie. older GNOME+GIMP toolkit or libraries). Unity 7 used GTK3, as GNOME 3 has for a ~decade now..
The moment you started using GTK3 apps on the GTK2 LXDE environment, the lightness of LXDE was starting to erode, as you ended up with GTK2 libs in memory (needed for desktop), plus GTK3 libs in memory (needed for your applications). That is one problem with LXDE (or XFCE when it used used GTK2.. you didn't provide release details; Xubuntu/XFCE is GTK3 from Xubuntu 19.10 onwards). Same applied back with Ubuntu-MATE/MATE only it ported much earlier; I used to love MATE on a pentium M laptop with 1GB of ram, dropped it when it ported to GTK3.
Next is GTK2 is dead upstream, no longer getting security patches, as were done only until
gimp (last GTK2 program) completed it's port to GTK3. So there are now potentially security risks with GTK2 as flaws aren't being patched. (new work is being performed on GTK4, GTK3 is where legacy maintenance now occurs)
LXDE -> LXQt history & toolkits, esp. related to Lubuntu
PCman (developer of
pcmanfm which was LXDE's file manager, but also handles the desktop itself) started porting it to GTK3 years ago, but there was a huge performance hit (GTK3 is heavier), so the work was paused, then a new port was done to Qt5. The Qt5 port was far lighter & had better performance... (PCman has blogged about this, it was long ago now)
The LXDE devs joined with Razor-Qt guys & are now the LXQt team. That's why Lubuntu switched from LXDE (depreciated except with 18.04) to LXQt. LXQt uses Qt5 libraries.
Lubuntu is I believe the lightest out-of-the-box of the Ubuntu flavors (be it LXDE using GTK2 or LXQt using Qt5), however if you're intended to use GTK3 apps, the advantage will quickly be lost, and it may not be best for you.
applications to be used
Consider the applications you'll use in your decision of what desktop. What toolkit do they use? as ideally you want it to be ones already in RAM, as they're used by the desktop itself thus resource hit will be less.
In particular attention pay attention to apps you'll use at the same time, eg. I still use Liferea (GTK3), Hexchat (GTK2) & Evolution (GTK3 MUA) on my LXQt system, the resource hit is greatest when I load the first of them, but hit isn't there for 2nd & 3rd app as libs are already in memory. My old 2009 desktop can cope because it has enough ram; so I can keep using apps I used back when GNOME2 was my desktop.
Further, there is taste. Not all of us think "chocolate" is the best ice-cream, so different desktops allow us to work in a particular way that suits us, so beyond the lightness or toolkits in use, we can choose one that meets our tastes, or weigh lightness against the other things we value (in our kepner-tregoe selection spreadsheets)
FYI: I used boxes with 1GB of ram to test releases up to and including Lubuntu & Xubuntu 19.04. I still do (18.04.5 was last in 2020-August), however most of my testing now involves boxes with more RAM. The amount of RAM you have will dictate how much care you take, with 2GB or less I'd plan ahead, with >4GB of RAM I tend to ignore it & use the applications I want to. The desktop I'm replying to you on, is a 2009 dell so it's old (c2q-9400) and my base is Lubuntu/LXQt, but I still use GTK3 apps I started using when GNOME was GNOME2 (& thus GTK2) as with 8GB I can afford the RAM hit. When using less ram devices though I'm much more careful.