Probably because it is, indeed, very bare-bones, and they hat to cut down quite a few things to get that 1-second startup (2 seconds on my Atom netbook in power-save mode to start and open a small default project).
That said, I don't feel much constrained even when comparing to the behemoth of Cubase SX-3 when whipping up an electronic track like this in Energy XT. The interface is very intuitive, customizable and thought-out, and the ability to switch between projects momentarily even on low-power hardware is quite liberating. As a VST host alone it might be worth its salt.
Again, the idea is that EnergyXT is fast. The selling point is that you don't have to wait for this and that to get to recording and production, and you can always polish off the details later in some other software if EnergyXT does not meet your needs. In that, I know no alternatives to it yet.
Download it from their website and give it a try; the only thing you can't do in the demo is saving.
As for Renoise/Ardour/Qtractor, Qtractor is the only comparable software (Renoise is a tracker, Ardour is not a MIDI sequencer). I haven't tried Qtractor yet, so I can't compare (need to reinstall Ubuntu first).
TL;DR EnergyXT trades off a ton of features for lightning-fast workflow, but likely still has most features you'd want from it.
Since my wife isn't here, I'm responding to this, because I think her experiences may be helpful to anyone considering using Linux for Making music. She has used Linux to make Music for 6 years now, and has gained a lot of experience, and even made a small tool for tweaking Ubuntu Studio audio related settings a couple of years ago.
My wife has used several iterations of Ubuntu Studio, for creating music compositions and recordings, but the latest one didn't work so well, and using the old one, was a dead end, because new versions of Ardour doesn't work on it anymore.
She is now using LUbuntu, and is quite happy with it. She has never been able to run with as low a latency as she does now.
She uses EnergyXT http://www.energy-xt.com, which according to her is more powerful than Steinbergs offerings. Although there is a Linux client, she uses the windows version with Wine and Wineasio so she can use her favorite VST instruments.
LMMS might be a nice free alternative, and AFAIK you can run VST instruments on it directly, without the Wine-asio driver.
LUbuntu is based on LXDE which is much more resource efficient than XFCE and especially Gnome, it is a full feature desktop environment just as XFCE, but runs much better, and there is no Pulse crap to bother with.
She uses the default 32bit generic kernel from LUbuntu, 32bit because it's easier with Wine-asio and VSTi, and generic, bacause it works great with the new kernel.
At 40 Euros, Energy XT is definitely worth a look.
Two killer features for me are 1-second startup and running VST plugins through DirectSound on Windows without ASIO drivers; it also has a decent synth built-in. Oh, and it runs on Mac and Linux, too.
Although not free, there's energyXT. It's relatively cheap at around $80USD. There's also the ReWire plugin sold separately to support Reason (it's incredibly useful), which is also cheap.
The people who make EnergyXT have that rewire vst that sets up a rewire or rewire-like link that's loaded in like a vst. I haven't tried it, but there have been some favorable reviews for it in the past. It costs 29 Euro. Here is the link: http://www.energy-xt.com/index.php?id=0115
Again. I haven't tried it, but it looks like it's a viable option for what you want. There may be someone on this list who's used it. I was hearing quite a bit about it a few years ago.
If you want to get into DAWs (Digital Audio Workstations) and still keep cost down then I'd suggest either Reaper or EnergyXT, they both have good trial periods and are dirt cheap to license if you like using them ($65 and $25 respectively, last time I checked).
Audacity is technically a DAW too, but it's not the most intuitive for performance.
You may want to learn more about VST (Virtual Studio Technology) plugins as well. They're basically components that add functionality to your DAW. One good resource is VST Planet, they have lots of good plugins for free or donation.
The Xfer OP-1 Drum Utility is a VST plugin, it can run in most DAWs.
Two come to mind you might want to look into: Usine Hollyhock MuTools MuLab
There's also my favorite EnergyXT - but that one is more music based than general audio production.