I've leveled two deception Assassins to 50(west coast and east coast servers. east coast had no queue when I started on launch day and I'm west coast).
The first one I leveled with Khem until Xalek joined me and while I never really felt underpowered, I hated having to Seethe for almost the full duration to get my companion back to full health. (Protip, dismiss & resummon or mount/dismount to insta heal them). I did have a bit of trouble with the Avatar of Sel-Makor, and I still haven't finished my class quest on the east coast server)
The second time through, I stuck with Talos as soon as he joined and the only fights I have trouble with now are pairs of elites or 3+ strong mobs. I rarely have to Seethe after combat for more than a couple seconds. It took one attempt to finish the class quest solo. Also, there's a Heroic 2+ quest on Belsavis at the Imperial Listening Station(I think that's what it was called) with a Champion difficulty Jedi with 70k+ health that was soloable using Talos.
I don't feel underpowered, but my only experience outside Assassin is a level 25 healing specced Operative.
Things to keep in mind(for soloing):
CC is important. Mind Trap on a strong/elite before the fight begins and you should be done with the rest of the group before Mind Trap breaks. Does not apply for droid fights.
Manage your force, important for interrupting(not for Jolt, but for other abilities that also interrupt).
Interrupt. Watch for the cast/channel bar. It's inevitable that some casts will get through, but with Jolt, Electrocute, Low Slash and Overload and proper force management, most casts won't get through.
Talos makes life so much easier than your other companions, you can focus on one mob in a group while the rest go after him due to healing aggro and pick off the rest or grab aggro on everything and make him heal you. If you've kept him geared, he can keep himself healed pretty well.
Stun & Run. Against melee, stunning them and creating a distance gap gives Talos time to catch up on healing during a tough fight.
Line of Sight(LoS). When fighting ranged mobs, again, stun and find somewhere to hide if the fight is close, It can't shoot you if it can't see you. Obviously you won't be able to do this when there are no objects to hide behind.