As a long term Empire General across several editions of Fantasy this is what I have found with the Hellblaster.
1). It is for the Empire a cheap and effective fear unit. When it fires off spectacularly it decimates the enemy line especially if you get it right. This alone for anyone who has faced one almost gives it double its value in points. It can be a great flank denial weapon, you stick it out on the flank and you will find unless your opponent has something to deal with warmachine specific targets they will be rather sheepish when it comes to moving within range of the thing.
2). As I have observed in 8th edition, people are still getting used to using skirmish troops. Reading any of the online forums that have a greater warhammer presence than here on heresy will soon lead you to conclude that skirmish troops are now either much classed as "useless" or "no longer useful". This is IMO a false premise. Players just have not become smart enough with them or have really read the rules well enough and understand how they really work. How does this impact your hellblaster? Simple... skirmish troops and scouts and hidden and anything that can be used to get close is a potential danger to the gun itself and the crew. Things like Dark Elf Harpies, shades, empire marksmen, dark riders without shields, light cavalry and night runners etc are all capable of taking out the Empire's warmachines when they are played correctly with little fuss. You need to consider that when placing the gun, and perhaps what you place around it.
As for Outriders.
I use them and Pistoliers often. I find them quite good as warmachine hunters and as worry troops. I play a classic Empire gunline anchored with a 40 odd strong unit of halberdiers in my centre. I want the enemy to come to me for classic anvil tactics. The Outriders and Pistoliers are great as they can go off and either take out war machines as the foe advances, or move about and bother the enemy. Be wary however that they are best suited to hit and fade tactics rather than sticking around, as they will be slaughtered.
Hope this helps you out.
Lexi.