Napoleon had a formidable army, and had designs to invade Britain. Those dreams were dashed when the Brits sank what little of a navy he had, and as I result, he decided on a grand continental campaign instead. Simply having the best army in the world will do you no good, if you lack the means to get them from point A to B.
My issue isn't whether it makes logical sense for Danaerys to invade at the current moment; it absolutely doesn't, she doesn't have enough troops (to take over an entire continent, even though she could certainly force surrenders through fear of the dragons), as you've mentioned she doesn't have naval support, the slaves she possesses probably aren't particularly well-trained, and she doesn't knowingly have any support in Westeros (and she most enter with the assumption that she will be facing the entire continent). My frustration is that her storyline is not one I find compelling any longer. The detachment to the main events, combined with the lack of knowledge she has of the land is a significant reason for this as it means that the characters she is playing off are primarily from her ranks (and as a result, generally have a similar relationship [I know there are sizeable differences, such as the relationship she has with Daario, Jorah, and Barristan for example; their outlook is primarily one of admiration, even if there are variations in the reasoning, and the personal relationship they have with her]), or characters introduced from whatever conflict she is currently engaged in who are fleeting (or have been thus far), presented in a manner which reinforces how fantastic Danaerys is (cruel slavers, and people obsessed with wealth in contrast to the kind, naive liberator that is Danaerys) creating a sense of repetition with her storyline and making her arc of 'slave-like (being sold to Drogo) girl who grows in power and confidence to become the culture-defying benevolent ruler/liberator/conqueror (depending on when she invades) of Westeros' rather bland (again, as she's been portrayed so heavily as benevolent, I believe the direction we're heading is some corruption of morals as ideals conflict with reality, and that will certainly ignite some further interest in her storyline if we go that way, but the isolation still serves as a heavy detriment). This lack of compelling situations she finds herself in (again, only my view for the reasons listed) is what makes the stalling (going from city to city to gather troops, naval equipment, experience as a ruler, and time for the dragons to grow further, so that she can begin her invasion at the most fitting point in the overall narrative [presumably shortly before or after Winter so that either her army and Westeros' is largely decimated before the White Walker threat, and then must deal with that, or so that her army is one of the final hopes of Westeros]) frustrating. Her plotline is ripe for rich stories (supplying of troops, defying of cultural norms, difficulties as a female ruler in a male-dominated world, training the troops, difficulties in sticking to her ideals in a world which renders this difficult, potential hypocrisy in her view of slavery, etc.), but with so many other characters (and almost nobody else in her location so she can interact with them) to deal with, in much more dynamic storylines, I don't think there has been enough time to make the most of these to make her compelling (in addition to some oversights like the lack of any mention of the usage of Astapor's naval fleet, which there surely are given that it's a coastal city).
Perhaps my view is not one that's shared, but so far, despite being able to recognise why her plot is occurring as it is, it isn't something I can get behind given the rather one-dimensional (this is overly simplifying it, I know, but I don't feel she has the depth of the other characters in her presentation due to the lack of interaction she has with a diverse range of well-developed characters) aspect to it since the first season.
