As the title states, my opinion of 6.0's iteration of SMN is entirely negative after living with it for a month. Not only is the job's damage (or firepower as they like to say in JP) underwhelming, it often feels like there is absolutely nothing one can do to actually improve it.
Additionally, with the help of a Japanese bug report, SMN's prog viability has been SIGNIFICANTLY reduced with the removal of the 2.5s cast time reduction from trances. SMN has gone from having raise parity with RDM to being entirely inferior, and in light of the damage situation I am not afraid to say that SMN in every way is now inferior to RDM.
I believe the primary issues with SMN are its damage, the fact that pet jank still exists and is a major problem for the job's raid buff, and the lack of decisionmaking one is capable of in the rotation. In typical illogical emotionally-charged rant form I am going to start with the third topic here as it is the one that is the most pressing.
Currently, the only decisions one makes as a SMN during normal gameplay are the order in which primals are done, which is firstly a question about movement and secondly a question about damage within raid buffs. Ifrit is ostensibly the least mobile primal, with Garuda a distant second, so most of this decisionmaking comes down to when you use Ifrit, and when you can gap close in Ifrit.
This is not enough. Period, full stop. Every single caster makes these sorts of rotation phase timing-movement decisions. The fact that this is the only decision SMN has to make about its rotation in full uptime (and honestly even around downtime this still is the case) really quite removes any form of accessible skill ceiling or optimization surface. In thinking about this problem, I have identified 2 issues that exacerbate it: the fact that all 3 of SMN's primals take roughly the same time to complete, and the fact that between demi summons, SMN basically just spends all the rites and favors they get from summoning the primals, removing any form of rotational timing decisions or other movement decisions from the job. These 2 issues effectively go hand in hand: together, they mean that SMN's rotation is entirely prescribed by the number of rites and favors you have to spend per primal phase, and the only difference between these primals is the amount you are able to move and the damage they end up doing.
This is simply poor job design. RDM has to make decisions about dualcast parity around OGCD timers, as well as banking melee combos and spending them either in buffs or for movement (not to mention accel charges and swiftcast for movement too). BLM gets instant paradoxes in ice phase, has procs that are instant and need to be timed properly, has a dot, has transpose lines to consider, etc. From my perspective as a longtime FFXIV player, SMN fails as a job because the skill ceiling is not there to keep high skill players interested.