Yes, creosote is problematic. If lignite coal coke gives more creosote than regular coal, then it will more profitable to burn creosote (in railcraft boiler) than lignite coal itself. Railcraft's config has "Fuel Per Steam Multiplier", which can be used to nerf boilers, but that is very blunt.
Check railcraft boiler calculator. It's outdated, but still quite correct. I tested using Railcraft_1.7.10-22.214.171.124.jar
But what happens if creosote bucket is equal to 3 charcoal?
Wood log: 20k HU.
Charcoal from it: 40k HU.
Charcoal + 0.25 creosote from it: 40k HU + 0.25 * 120k HU = 70k HU. (75% increase)
Coal: 40k HU.
Coal coke: 80k HU.
Coal coke + 0.5 creosote: 80k HU + 0.5 * 120k HU = 140k HU (also 75% increase)
Lignite: 10k HU
Lignite coke: 20k HU
Lignite coke + 1 creosote: 140k HU (700% increase... time to go digging$$$)
Ethanol needs a buff or biogas needs a nerf. I recently tried to calculate how much HU can I get from the same amount of biomass. Biogas wins by very approximately 10 times.