There's a certain balance to be had:
- Too little hand alternation (e.g. QWERTY) can cause strain when typing certain words.
- Too much hand alternation (e.g. Dvorak) can be slightly slower and more error prone.

The bigger issue with Dvorak is that its design overemphasizes hand alternation, thus sacrificing other aspects of the layout design.

I think Colemak strikes a good balance. If you go to extremes, it's better to have too much hand alternation than too little.

See also the Hand Alternation page

To clarify: In your example, the ST and ARS in 'stars' are simple inwards rolls on Colemak, making 'stars' a mixed roll or a composite of two rolls. Both these rolls are considered good, and the word is good to type on Colemak.

As SteveP said, the strongest fingers perform much better and more effortlessly for rolling. Furthermore, inwards rolls towards the index fingers are considered a bit easier than outwards ones towards the pinkies, and these again are usually easier than mixed-direction rolls like 'rast' or 'ion'. The word 'you' on Colemak is a well-known example of a weak-finger mixed roll that bothers people with weak pinkies and codependent ring fingers.

The whole string "in my home" isn't considered a roll just because it's on one hand, even if it contains the bigrams IN, MY, HO and ME that roll pretty well individually. Since the phrase consists of four separate bigrams, I at least would not call it a roll. A right-hand roll would be something like 'neon', which is a mixed-direction roll. When it gets punctuated by same-finger bigrams, more than one change of direction or anything else that breaks it up, I wouldn't call it a roll anymore. So I don't consider a word like 'lone' a roll: That's two bigrams, since it doesn't roll as one entity.

You may then ask why I still considered 'stars' to be a roll? Well, I just felt that it rolls as one entity because it's all on the home row. But maybe others feel differently about that? The safest term for 'stars' may be a composite of two rolls.