Whether to come back is a tricky one, depends on what you want from the game on your return...
Populations everywhere are lower, and so activity can be harder to find - the community is mostly really helpful, many good folks will do all they can to being you back up to speed on all that has changed. If you want to 'optimise' characters, it's much more complex than you would remember from years back - items, their properties and interrelations are far more important now and have many more variations. To simply get good enough stuff to do most things is not that hard, but you will need a lot of reading and advice if you want to go much further than that for top-end PvM, and especially if you wish to be more involved in PvP. RP is scattered a lot now, smaller communities on most shards (and on some sadly almost none) but again small pockets can be found if you ask around in-game and on the shard forums. The game still has a lot to offer and a lot for people to enjoy.
However .... the nastier element in the game - the scammers, cheats, event disrupters and so on - are allowed pretty much free reign nowadays, and for all the words you may hear from the developers, there is no genuine or effective response from 'support' - and apparently it is 'too hard' for the existing company to ever provide. Similarly, the account management pages are sometimes almost impossible to navigate and use, and the availability of things from the Origin store (the replacement for the UOGameCodes site you might remember from way back) is hopelessly variable depending on which region their automated systems decide you 'live' in, and frequently we see incorrect codes being given out by the site, or reports of people being billed for failed transactions. Again, despite endless promises these issues will be resolved, it's been literally years of no progress on them and we're still being told 'improvements are coming'. Many old bugs remain, despite endless reporting of them - to the point where it's more usual for players to invent workrounds than to see things actually fixed.
Simple answer - if you can ignore the shortcomings of the company who own the game, there is a lot of enjoyment to be had from it. If you want professionalism, well written code, and proper in-game and out of game support for your money - look for a different company and game.