As a minor nation, Bulgaria suffers from all the problems associated with one. It has some limited bonuses as regards to its units (which train faster and deal more damage to buildings), but its unit selection is as questionable if not worse than that of France.
While Bulgaria may have a rather impressive lineup of German-produced aircraft, and vehicles, it generally is a case of "too little and too late", given that most weapons provided through foreign aid come in one age later as opposed to those used by their producers.
Bulgaria may be a good early-game winner as long as it can mount a successful blitz that, failing to defeat the enemy, at least destroys a good part of his economy. That will be no easy task, given that Bulgaria has no proper tanks to speak of in the early game. In the mid-game, what possible weapons there may be are few in number to make any significant effect, and in the late game, a lot of factions will have more advanced weaponry which can even take away the advantage armoured units may bring to the field, and even so the many foreign-produced units that Bulgaria may obtain are often of questionable quality themselves when not outdated.
Bulgaria will sign on to no alliances but will remain seemingly non-belligerent, unless you go to war to Greece, or the Axis is formed. Only war with Greece will encourage Bulgaria, and if Greece is attacked by any nation Bulgaria immediately becomes belligerent but will not attack anyone else except Greece.
(NB: German and Italian units available only with Fascist alignment)
- Anti-armour: Solothurn S-18/100 [3-4] => Panzerschreck [5-6]
- Shock Troopers
- Tankette: L3/35 (see Spain)
- Light tank: V6T  => LT35 [3-6]
- Medium tank: Panzer III  => Panzer IV [4-6]
- Romfell armoured car  => Sd.Kfz. 222#
- Fighters: Albatross C.III  => Arado Ar 68 [2-3] => Focke-Wulf FW 190  => Messerschmitt Me 262 
- Special: Junkers JU88# 
- Barrack Units do 5% extra damage vs buildings for each Age plus Military research
- Barrack Units built 10% faster, 7% cheaper,for each Age plus Military research
- Cheaper Farms and Trawlers