These guys seem honestly useless, and most CR players I see (myself included) just end up building grenadiers and totally neglecting the mainline replacement of this BG.

Let's break them down vs grenadiers
Advantages of coastals:

• Can repair structures and build structures
• Slightly lower initial cost

Disadvantages of coastals:

• Terrible combat performance (awful fire on the move and non-cost efficient trading against other mainlines)
• Higher upkeep cost compared to grens
• Way worse vehicle snare that also comes significantly later (gated behind T2)

If you build coastals instead of grens you WILL get pushed off the map. I'm not sure if the intent of the BG is to still stay gren focused, but it would be nice if coastals could at least hold their own.