Yes, when it comes to BW, its mechanical difficulty is on a whole different level and that's one of the cited reasons. In fact, from all available RTS games out there, few even come close. StarCraft II is probably the only game that might just slightly touch BW's mechanical difficulty. However, even there, it's a really hard comparison to make and doesn't take into account the unit selection limit which was a conscious design choice on the part of BW's creators. As a simple example: it's mid game TvZ and you make a small group of mnm and move out onto the map to be aggressive. You stumble onto his mutalisk flock and get caught up in an engagement. You have to make a decision here. You either go back to macro and risk losing the battle without doing much or stick around and watch your money pile up and your future army, assuming he doesn't counterattack, be smaller than it should be.
Of course, if you are someone who's gotten into the rhythm of macro and have learned to multitask fast enough, you can at least quickly make units and go back to your army to prevent total annihilation (if he commits) and do something. Against an opponent who is on a similar skill level, you both won't macro optimally and you both will make odd decisions at times. Other compounding factors are army control, understanding the various matchups and decision making. These are all just as important and play a large role in any BW game.
You can be the best macro player in the world but if your control is shitty or you keep forgetting to scan and run your mnm into hold lurkers (most painful thing ever ;; ), you will more than likely lose to an "inferior" player. If you don't know the ratio of units, you can have more marines and less medics which means overstimming will be a bigger issue (it always is) or you can go heavy on tanks and less vultures which can be problematic in TvP depending on your opponent's composition. It's all about striking the right balance, making the best decision possible with time constraints and that comes with experience (playing more games, vods, etc...). All of these things in combination are as much of challenge for one newcomer as another.
In other words, while BW can be called a difficult game in a vaccum, due to having to accomplish a lot of different tasks, it's really not that rough of a game from this perspective since you and your opponent will always have a lot of small things that you both need to work on. This is where the fun comes in. Fun in seeing yourself improve constantly. All of the mini reminders that what was once impossible for you years or even just several months ago is now possible. It's extremely easy to improve if you've got the right mindset.
Your macro sucks? Queue up units or make more production structures or learn to be more aware of map pings. You forgot to build pylons/depots/overlords? Make a couple with each cycle or build 3 at once if you can spare it. Your army control needs work? Practice offline using all those micro maps. Multitasking is lacking? Learn to tab back to your army after each production cycle or use the spacebar whenever you get an alert (this can help with macro too). It's mostly a matter of learning how to balance everything and get better at the game while you also have fun with your opponent. This doesn't even factor in one of the amazing parts of the game imo, the mind games. If you find a player that plays as well as you do and play with them over and over, you learn about their tendencies and in turn, you can mind game them. It's a thrill when it works!
To illustrate some of the points above, here are two games that were played recently against an opponent whose skills are quite similar to mine. The first game was played on a map called Neo Electric Circuit. It's essentially a really large macro map with a ton of bases that can be secured and a mineral-only back expo. If you watch the game or parts of it, what you'll see is some piss poor macro at times. Possibly worse than my opponent because let's face it, banking close to 5k on 3 bases is a ton of resources, but what you'll also see by the end is a great victory despite that. It was about the right decisions at the right time and being aggressive just enough to take it:
Fast forward to this second game on another map called Bloody Ridge. The map is considered great for terrans to mech on. Being someone who loves bio, I decided to stick to bio. In comparison to the first game, my macro was on a totally different level this game. I had more production structures and I almost always had a large army to use after failed engagements. I was killing drones and (in my mind) being aggressive enough etc... Thing is, this game was lost despite cleaning out 3 bases worth of drones. Mistakes made in army control and decision making despite "decent" macro all lead to the aforementioned result:
In closing, BW is not easy and that's where the fun is. Once a player frees themselves from this notion and focuses on finding a similarly-skilled opponent, BW becomes much more dynamic, much more fun, much more welcoming and much easier too. For anyone who made it this far and might want to watch some more fpv, here's another 2 fun games that I decided to upload showing just how much different each game can be: + Show Spoiler +