I'd say OST if most difficult faction to play because it's best design. No shitty bs, no freaking OP blobs etc etc and becasue if this, it's hardly melted by allied factions.
Ost is behind because they need about 1 minute to put the first fighting unit on the field because Pios have to build T1 and only then they can bring Grens on the field, a squad with only 4 man that is only good behind cover and in long range.
Because of the time needed, many Allied players put their squads into choke point buildings where it is very hard for Ost to get them out because either you have to spend mun for a flamer (no teller mine against light tank rush) or get a mortar (even less capping power). Getting a fast MG is imo the only viable solution, but the capping power still is very low.
Sov: Can get conscripts from T0 at the first second (main battle unit)
USF: Can get rifles from T0 at the first second (main battle unit)
OKW: Can get volks from T0 at the first second (main battle unit) + Start with Sturmpios
UKF: Starts with a IS squad (main battle unit) and can get the second from T0 at the first second
Ost: Starts with Pios (weak unit with little fighting potential) and needs T1 to get the main battle unit
While Ost gets the first grens, all other factions usually have at least 2 squads of their main battle units, which are simply better in the early game (rifles, infantry sections) or have a higher amount of soldiers (2x conscripts vs 1x grens is 12 men vs 4).
The Ostheer is always about a minute or two behind and their single purpose is to react, making it by far the most defensive faction in the game right now (disregarding UKF sim city). There is very little room for Ost to pressure the opponent unless you are clearly playing better. The lack of Ostheer's close combat unit (Pzgrens come too late and can't engage because of high lethality) and no light tank makes them even more defensive.
I like that playstyle very much myself, but with the introduction of the British ultra sim city commander, Ost plays against a more aggressive faction that has also better defensives, leaving little to no space for counters. A mortar loses badly against a mortar pit which can be repaired automatically and braced. A well placed bofors can only reliably countered by a mortar HT which is only available in 2 commanders, from which one sucks and the other one is premium (spearhead).
The MGs, the real backbone of the Ostheer (not the grens IMO) are simply put out of the game against the helms deep commander that is defensive/aggressive at the same time because of counter artillery and mortar pits, both with immense range. The same thing happens with your Paks, leaving the Ost player no reliable counter against british tanks except StuGs (but then you can't counter the superior British inf). If the Brit manages to cover two VPs with his Mortar and Bofors, then it is extremely hard to do anything as Ostheer.
So in my opinion Ostheer vs that particular Brit pay2win commander is the hardest setup right now.