from my point of view, SA is great with element dmg. + also faint phial, i find other special phial useless

SA is a very versatile weapon. Lots of people find it hard to pick up. Below are solely from my own experience with SA on Tri.

Axe Mode: High and long Reach, High Damage. Slow Attack. Good for knocking mobs over. Also very mobile as you can move quickly. Useful if you need to chop off high tail or chase after mob. Phial is recharged in this mode.

Sword Mode: ESP. Does not bounce. Very fast attack compared to Axe mode. You will however be rendered immobile. Movement speed is comparable to GS. Very good for breaking parts, e.g., Horns, Wings. Unsheathe attack is faster than Axe Unsheathe. Phial is drained in this mode.

I usually do not sheathe the SA after drawing if possible. I will move mainly in axe mode and do major damage with sword mode. You will need to learn to combo from sword to axe to sword and vice versa to get the best out of SA. All vertical attacks of SA does major damage.

For other phials, e.g. Status phials, it is actually pretty useful if you can get in a full burst (what you guys call boom boom here). The burst attack can pump in a lot of Status Dmg to the mob in a short time if successful.

Hope this is of help.