Played the tutorial and a few challenges in Starcraft 2 and on a hunch I searched the net if there is unit veterancy. Turns out there isn't. There's a mod for it, but those are iffy. And the absence of unit veterancy in an RTS is an instant turn-off for me.
I always like to create a small squad, made up of only a few units of each type and carefully fight the enemy, by attrition or hit and run, making good use of waypoints to sync attacks and retreat and since I can do none of that, gameplay becomes unlikable with the lack of a formation, stacking orders to execute on release...etc. making things unnecessarily more difficult. And there is no incentive for me to value every unit, like I always do, since I'm merely required to exploit some counter-system, pitting units against their weaknesses, which still results in loss, no matter how well I execute attacks, which is annoying to me.
I played through C&C 3 on normal difficulty, losing only one Pitbull, in the final mission, because I wasn't paying attention to a part of the map.
And what's really sad and pathetic with RTS games in recent years, is that back in the 2000s, in Red Alert 2 I could enter waypoint mod, issue a movement and attack pattern to units and have them start ONLY when I clicked the planning-mode again, which allowed me to properly execute simultaneous attacks with multiple selection groups on multiple targets, which was an immense help in ambushes and focusing fire when attacking defenses.
I haven't seen that option since. In C&C 3 I could order groups of units into a formation.
The most recent game that came close to having such old, but excellent elements was Homeworlds: Desert of Kharak and I made it through the campaign with only having half of my max fleet size, and almost every unit was on max veterancy, but that didn't mean they were indestructible. Not, at all. Which was a thing I liked in Red Alert 3, that heroic rank units could be easily killed of if they went up against a unit they were weak against and which was strong vs. them. That's balance. Or simply, it's just a way of not making max rank units god-like, one-man armies against absolutely everything.
To me an RTS that lacks those elements haven't had much thought put into it, gameplay-wise. The story, graphics, characters, UI can be all pretty, but if it plays like crap, lacks luster, with no reward in valuing every unit, where strategy boils down to almost mindlessly steamrolling through the enemy, and even when I exploit weaknesses and still suffer needless losses that could be avoided by having better unit management options, then the game isn't worth the time.
I recently tried the old Homeworld series, as well, and while I could carry units over to subsequent mission, there was no unit veterancy, and the only way I could prevent losses was by outnumbering the enemy 10:1, which meant having nauseating amount of eyesore units on screen. Nuh-uh.