Imo. Jack of all trades master of none but is often times better then a master of one.
Flexible in a wide range of metas with a wide range of builds and can use a wide range of runes. you can help adc win lane, you can impact jungle and mid very easy as well.
A well cast R can make a game and a bad cast can break one.
I play him because his E and innate tanky stats enable some deep warding at a lot lower risk then many other support picks. his E enables some great gank paths that rarely get used because idiots wont treate terrain as a path with a bard on their team.
he has fantastic outplay power most the time and a very good scaling passive if your allowed to collect it. Good enemys will react to a roaming bard. Bad teammates will see your not near them but 1v2 then bitch that your roaming seemingly completely unware that if they just waited for 5-12 seconds youd be back as your only in the river to get some MP and drop a ward.
I really like
on him and using Item CDR runes. can drag out a game to late, can use the portal as vision steroid by putting it in jungles. the portal gives vision, the lings give vision. Its great bait because you can R some one trying to break it then E into them and in jungle snagging a stun is quite easy.
one of the most hilarious things is a lot of players are unaware its coded as terrain and they can get stunned by Bard Q if its behind them. And even if they know it gives you an option if some ones positioned well to not get stunned. you can get a lot of kills for your team with a good R, E, Zzrot drop and Q outta R.
Plus you can just drop it in a lane and group. They send some one to stop the ZZ push and you got a nice window to ulti for a 5v4 that starts with enemy being hard CC. They don't? you can just disengage with R and your teams gonna get free damage on an obj. Can BD low hp towers by setting it up in a jungle behind towers.
And its protection from everything but true damage.
W is a terrible on demand heal and functions mostly as a MS steroid in teamfights. Its ok when you have 3 of them down. I typically grab W first so I can give mid lane or my jungle a bunch of HP. often times 1 in jungle, ward for an invade and 2 in mid then back to make it to help leash.
I don't like flat ap bard builds. Other options are better. AP bard is again a weak heal. AP bard is cheese that can be done with pretty much any champion, most of whom have better AP scaling and stronger damage.
Pure tank bard is awfull because your not really looking to front line and you don't have much in your kit to keep ya up.
So I tend to go for item activates.
are my core. AD seems weird but you get more damage from the Flat ad on Botrk then 100AP provides his passive, it only triggers off autos after all. Using the Meeps to slow a tank is a common move, so % damage helps soften them up. and oviosuly you have an on demand slow, MS boost and damage from the active. Life steals not wasted completely thou its the weakest gain on the item. The as seems weird but it enables you to overlap meeps slow duration so you end up chaining a increasingly strong and long slow on front line targets which is helpful for peeling things away from back line.
Then your left with pretty much any item that might not have the best gold effeciancy but solid actives. Massively reduced CDR on item actives is pretty nice. Need engage?
. Disengage?
. DPS glass cannon and needs assistance?

At any point you can pick what direction you want to build in response to the match, have a strong team fight pressure. its good into a wide range of metas and really I only struggle when games end to quickly from snowballing and bad vision control.