Riflenades are less effective at clearing buildings, but can do so at range. Molotovs are more effective, but are short-range (IE risk of model loss) and require teching. It's balanced. Get a flamer pio if you want reliable anti-garrison. Or the new, better mortar once patch hits. Or FHT. All these are available within minutes of the game starting.
IMO, buildings should still become more vulnerable to small arms fire. I don't really like the idea that they need to be hard countered. But it's Relic's philosophy as of now. And it makes playing Semois supremely annoying as both factions.
Right, small arms fire should have more of an effect on buildings. The problem is that in the first couple of minutes, I can't very well rush a flamer pio (even if I spend my 1st 60 munis on it, pios are very easy to push back since they are low HP/armor) and I can't rush a mortar either since mortars that early tend to get outflanked and pushed back quickly. My only chance to fight these buildings is getting in one myself, back-capping, or heavily defending area with a lot of squads and ceding map control in the process.
As such, grens/cons, snipers, and MGs need to perform better against buildings or the pioneer/Combat Engineer flamer needs to clear buildings faster. Small arms fire are just too inefficient to try to use against buildings at the moment.