It is not something Garland likes, but it's this minor squabble, so to speak. The same goes for the Reach, albeit it differently- it's defense in depth, not defence in trapping. You could walk in, genuinely, just run through half of the Reach but you'd be stopped dead in your tracks by an assembled host, that would basically offer so much resistance, you'd have to turn back. It's number that wins wars, and when you attack, you can't do guerilla tactics very easily- but the Reach can reclaim it's own lands very, very fast, owing to a greater knowledge of roads, and the fact that there is not much to hide in. You can't blanket a whole force in a rosefield, you can make a guerilla strike, but not much of one, so everything else is forced to be placed right into the open, unless you pick an area of the Reach that has hills or is covered in conifers, and even then, the sheer size allows for flanking, and for forces to just spread wide. Exposure is good- and in that vein, that's how the Reach works, giving them that defense in depth, slowing down any enemy force to a halt before they manage to begin sieging. It definitely is a big difference- the Reach does big, loud, massive battles well.
The Dornish are good at guerilla warfare, that is one certain thing that can be said, something that The North have kinda implemented themselves with their Ranging Patrols. The Reach is all about their huge armies, and as I briefly explained, there is a concept of the Tyrell Retinues- or basically, semi-professional soldiers, they are no legion, but they are this idea that there has been a development in the armed forces of the Reach. I took the idea from both in concept ideas of foreign armies that supported Dany, as well as the Reach's own approach to enforce it's political soft power, each formed from both local levies as well as men who are paid to basically train and maintain a garrison force. There's one in Tumbleton deployed to King's Landing, and there is one in Torrentpeak, spread across the border as a border guard, and to stop Dornish raiding, though the minor lords are generally so far from Highgarden, they raid into Dorne sometimes with bad effects. It is not fun for anyone really, it's kinda a blight for Garland, and no doubt, for your Princess.
But yeah, apart from those roughly 20,000 mixed Retinues, from Knights to Pikes and Archers in that force, there's about another 60,000 men that can be called from the banners to war, that are NOT Retinue men. It is a classic sort of assembly of an army- it is Westerosi, through and through. Reach Knights at this point rival the Knights of the Vale, maybe not in training but are equipped as probably the richest Knights of the Kingdom due to The Reach's relative prospertity, which support pike and poleaxe forces (something that has had a huge backing from Gregor, pikes basically being the death of any charging horse, if used appropriately), as well as longbow-equipped archers, using a mixture of bodkins and broadheads. Crossbows are an interesting thought, but considering how much like England and France the Reach feels like, it didn't feel right for me to have them as an en-masse as of just yet, being something that I think would be far more appropriate in a city-deployed retinue rather than on a battlefield. If the Reach in the War of the Five Kings was a force that had a typical Medieval look, at this point, it is almost a Mid to Late Medieval army, mixing a funny combination of heavily plate and mail armoured Reach Knights with lances and spears, longbowmen, pike and poleaxe infantry and in with all of that, swordsmen from place to place. I like to think of it as a classic Westerosi offering, but with what a century or two of military academic progress would look like in The Reach, to satisfy political and actual combative needs- there is no exotic way that they fight, no Dornish spearmen or no crossbows or horse archers, it is bogged down in this idea of seriously hard charges, interspersed with arrows, any counter attacks dealt with by pikes. It's traditional, but it's advanced traditional, that is what I would imagine it to be- the only way forward for the Reach if it wanted to develop anything further would be to actually formalize it's forces into a professional army, though that would reduce their numbers by about two-thirds, I would imagine, and be hugely costly compared to what they have now- though it would be one that would be fucking terrifying, what with it being paid to basically be at the height of it's military prowess, and would be perfect if the Reach was far more involved in war than it has been.
I imagine that the Tyrell doctrine for war is basically a mixture of small-scale rapid deployment, being able to beat any massive host to cities such as King's Landing and to defend the wide frontier with Dorne and the Shield Islands, whilst if you want more than that, you'll need to wait if you want to go fully to war- and while it pisses vassal houses off, it gives an extraordinarily large army. It's not guerilla warfare, it is the traditional way of doing things- you march over plains, you will fuck anyone over, you march into mountains and heavy, heavy forests, unless the force is led by commanders such as the Tarlys who would almost definitely know the Dornish commanders inside out, they will get pretty bogged down and suffer attrition while pushing forward. Remmeber, Dorne beat the Targeryens back, they were never conquered by any military force- and for now, the same pretty much stands, that idea of Legions and sort of this hugely attritioning warfare, from the sands, is something I like.
This is why I love GoT- it's the contrasts, changes and ways that things are done in different Kindoms, it's just so interesting to observe how some take an approach to war in what particular way with what they have at their disposal- ie. the Reach's army is not going to be guerilla when it is that large, it is not possible, so they are good at what they should be good at.